Keep track of persisted ban byte usage, and it's fragmentation.
Fix the length recorded for truncated bans (it should be the length of
the header, not 0).

Add a test case for reload of truncated bans.

Fixes: #1266
Mark the ban_magic as gone also in the persisted ban spec, so it can
be recognized as such on reload
Honor the persisted GONE flag on reload of bans, and keep the
duplicate counters consistent on reloads.

Add a test case for this.

Add a section on bans and offline silos in the docs.