Add support for configuring timeouts, stats file, and listener

[jacobneiltaylor]

This PR adds the ability to configure various elements of the Anchor server using environment variables.

Specifically the following settings can now be tuned, with existing defaults preserved:

• Listener address: ANCHOR_LISTENER_ADDRESS
  • Defaults to net.IPv6unspecified, which replicates the existing default behaviour of listening on all available IPv4 and IPv6 addresses.
• Listener port: ANCHOR_LISTENER_PORT
• Stats filename: ANCHOR_STATS_FILENAME
  • All filenames are fully resolved prior to handoff to the server module.
• Heartbeat interval: ANCHOR_HEARTBEAT_INTERVAL_S
• Inactivity interval: ANCHOR_INACTIVITY_INTERVAL_S

The fully resolved configuration values are emitted at the beginning of execution to aid server operators.

Additionally, this PR cleans up the Go.mod file (removed improper // indirect annotations) and adds an initial log line emitting the Go runtime details.