Add a "recent server state transitions" list to the server_info rpc command

[seelabs]

Summary

Right now the server_info rpc command returns a summary of server state transitions (full, syncing, tracking, etc). We should also return a list of the last 32 transitions, along with the timestamps showing when they occurred.

Motivation

When diagnosing server issues it can be useful to know more detailed information about recent state transitions. If the server has been running for a long time then problematic recent transitions may be hidden by the large amount of historical data.

Solution

The server should keep a circular buffer and track the last 32 state transitions and return them with the server_info rpc command. (32 seems reasonable, but of course we can discuss other sizes).