Flush rollback log more often

Flush rollback log more often

Flushes the buffer of rollback actions that wait to get saved in two more situations:

1. Flushes in the destructor of the rollback. This makes the server not
forget the last < 500 rollback entries when it shuts down.

2. Flushes the rollback when /rollback_check is invoked. This is neccessary
as otherwise it leads to confusion if users want to test the rollback functionality
by placing a node and then executing the check on it, or if the actions were
very recent out of other reasons.