fix race that could fail to persist a ban

DumpBanList currently does this:

• with lock: take a copy of the banmap
• perform I/O (write out the banmap)
• with lock: mark the banmap non-dirty

If a new ban is added during the I/O operation, it may never be persisted to
disk.

Reorder operations so that the data to be persisted cannot be older than the
time at which the banmap was marked non-dirty.