Unknown signer issue resolved by resetting the database

[obycode]

There is some scenario in which the signer gets into a state where it is unable to keep up with the chain. When exhibiting this behavior, the signer is showing excessive disk access activity. If the database is deleted and the signer is restarted, the problem goes away.

The issue does not appear to be related purely to the size of the database, as we see healthy signers with databases larger than that of those that got stuck behind.