troubleshooting rippled ledger gaps

[mDuo13]

Ledger gaps can occur if:

• server has insufficient resources (especially disk I/O)
• server was recently restarted
• server lost network connectivity temporarily

We should add a guide to the troubleshooting docs to explain these possibilities and suggest some ways to narrow down which one may have occurred (for example, checking the uptime and the state transitions in the server_info to confirm that the server was online and available continuously, checking I/O load & counts) and to determine why. For example, check other network monitoring to see if it was a generalized network outage; check available disk space; consider whether there are periodic or occasional jobs like backups running on the same server that may compete with rippled, etc.

There may be some overlap with existing docs on troubleshooting and server not syncing.