This adds set reconciliation for ATXs.
There are per-epoch syncers, with lower FPTree depth (16 by default)
used for older epochs and greater FPTree depth (21 by default) used
for current epoch.
Both active syncv2 and passive (server-only) syncv2 are disabled by
default. It is possible to enable syncv2 in server-only or
full (active) mode.

It turned out that sync interactions are happening rather quickly,
and thus it is not really practical to try and begin handling arriving
keys (e.g. ATX IDs) during sync itself. Moreover, on-the-fly key
handling was actually only used to register peer IDs for each ATX ID
to fetch the actual blob from, and that can be done in the handler's
Commit() method just as well.