When the active quorum controller encounters an "unexpected" error, such as a NullPointerException,
it currently resigns its leadership. This PR fixes it so that in addition to doing that, it also
increments the metadata error count metric. This will allow us to better track down these errors.

This PR also fixes a minor bug where performing read operations on a standby controller would
result in an unexpected RuntimeException. The bug happened because the standby controller does not
take in-memory snapshots, and read operations were attempting to read from the epoch of the latest
committed offset. The fix is for the standby controller to simply read the latest value of each
data structure. This is always safe, because standby controllers don't contain uncommitted data.

Also, fix a bug where listPartitionReassignments was reading the latest data, rather than data from
the last committed offset.