db: better handling of background errors

When a background error occurs during a flush or compaction, Pebble currently just logs the error and attempts to retry the operation indefinitely. Depending on the error which occurred, this isn't terribly helpful. If the error was due to lack of disk space, a retry loop can be helpful. But if the error is due to a logic bug in Pebble or some sort of corruption it would be better to place the the DB in a read-only mode. This is the strategy used by RocksDB.

Jira issue: PEBBLE-181