adopt go1.13 stdlib errors

update module path

add isc license

add go1.13 to go.mod

add HTTPClient interface

.

add remote addr to heartbeat log

use cleanhttp to fix keepalives file descriptor bug

wip

fix deadlock, simplify service error handling

store host, role for logs

add host, role info to logs

fix data race

update github.com/pkg/errors to v0.8.1

fix lock contention bug

improve logs

remove database, keep state in memory

add client timeout

Expect a client to respond with 200 OK in less than 10 seconds, even if
the job may take much longer.

fix linter warning

add github.com/pkg/errors checksum to go.sum

fix context cancel bug