wal heartbeat: allow overriding query with env variable

[serprex]

To enable heartbeating, set these two environment vars:

  PEERDB_WAL_HEARTBEAT_QUERY: ${PEERDB_WAL_HEARTBEAT_QUERY:-UPDATE _peerdb_heartbeat SET last_heartbeat = now() WHERE id = 1;}
  PEERDB_ENABLE_WAL_HEARTBEAT: ${PEERDB_ENABLE_WAL_HEARTBEAT:-true}

[CLAassistant]

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[Amogh-Bharadwaj]

Should we also make the frequency of heartbeat an env ? @serprex @iskakaushik

[serprex]

I think that would be better as a later PR when needed, has to touch at temporal cron setup

[Amogh-Bharadwaj]

Functionally tested, updated description