Don't accept any IP as a proxy by default

It's too easy to setup a reverse proxy which doesn't support the PROXY
protocol, or lets the X-Forwarded-For header fields pass through.
Disable this by default.

To restore the previous behaviour, add `accept-proxy-ip localhost` to
the config file.
Upgrade dependencies
Switch to go-scfg

We don't use child directives yet, but likely will in the future.
Add message store abstraction

Introduce a messageStore type, which will allow for multiple
implementations (e.g. in the DB or in-memory instead of on-disk).

The message store is per-user so that we don't need to deal with locking
and it's easier to implement per-user limits.
Switch to sql.NullString

Not really better than what we had before, however new contributors will
maybe be familiar with it.
Switch DB API to user IDs

This commit changes the Network schema to use user IDs instead of
usernames. While at it, a new UNIQUE(user, name) constraint ensures
there is no conflict with custom network names.

Closes: https://todo.sr.ht/~emersion/soju/86
References: https://todo.sr.ht/~emersion/soju/29
Add id column to User table

We used rowid before, but an explicit ID column is cleaner.
Add support for the extended-join capability

This simple implementation only advertises extended-join to downstreams
when all upstreams support it.

In the future, it could be modified so that soju buffers incoming
upstream JOINs, sends a WHO, waits for the reply, and sends an extended
join to the downstream; so that soju could advertise that capability
even when some or all upstreams do not support it. This is not the case
in this commit.
Add support for the PROXY protocol

IPs whitelisted in accept-proxy-ip can now use the PROXY protocol to
indicate the original source/destination addresses.

Closes: https://todo.sr.ht/~emersion/soju/81
Set Server.AcceptProxyIPs
7310f67f — Hubert Hirtz a month ago
Make DB.DeleteChannel take the channel ID

... to allow the caller to correctly do any necessary casemapping.
42828d68 — Hubert Hirtz a month ago
Make sure that WebSocket messages are valid UTF-8

... by replacing invalid bytes with the REPLACEMENT CHARACTER U+FFFD

This is better than:
- discarding the whole message, since the user would not see it...
- removing invalid bytes, since the user would not see their presence,
- converting the encoding (this is actually not possible).

Contrary to its documentation, strings.ToValidUTF8 doesn't copy the
string if it's valid UTF-8:
Don't update history when msgID is empty in appendLog

Log files don't record all message types. If the message isn't inserted
in the log file, don't update the history cursors for downstream
Fix panic in downstreamConn.sendNetworkHistory

This panic happens when sending history to a multi-upstream client.
sendNetworkHistory is called on each network, but dc.network is nil.

Closes: https://todo.sr.ht/~emersion/soju/93
Fix downstream PING argument handling

The PONG message should have these arguments:

- Our server name
- The PING message's source name

Closes: https://todo.sr.ht/~emersion/soju/92
Allow '/' in nickname

This allows to specify a network name in the nickname.

Closes: https://todo.sr.ht/~emersion/soju/91
Update dependencies
Nuke in-memory ring buffer

Instead, always read chat history from logs. Unify the implicit chat
history (pushing history to clients) and explicit chat history
(via the CHATHISTORY command).

Instead of keeping track of ring buffer cursors for each client, use
message IDs.

If necessary, the ring buffer could be re-introduced behind a
common MessageStore interface (could be useful when on-disk logs are

References: https://todo.sr.ht/~emersion/soju/80
Introduce loadHistoryLatestID

This loads latest messages from logs up to a given message ID. This is
similar to the IRCv3 CHATHISTORY LATEST command [1].

[1]: https://github.com/ircv3/ircv3-specifications/blob/0c271a5f1df4f93b5ae4c7597422821c40a8cfeb/extensions/chathistory.md#latest
Introduce internal message IDs

For now, these can be used as cursors in the logs. Future patches will
introduce functions that perform log queries with message IDs.

The IDs are state-less tokens containing all the required information to
refer to an on-disk log line: network name, entity name, date and byte
offset. The byte offset doesn't need to point to the first byte of the
line, any byte will do (note, this makes it so message IDs aren't
necessarily unique, we may want to change that in the future).

These internal message IDs are not exposed to clients because we don't
support upstream message IDs yet.
Replace networkHistory.offlineClients with clients

Keep the ring buffer alive even if all clients are connected. Keep the
ID of the latest delivered message even for online clients.

As-is, this is a net downgrade: memory usage increases because ring
buffers aren't free'd anymore. However upcoming commits will replace the
ring buffer with log files. This change makes reading from log files