Set Mongo connection pool size better.

Fixes (for now) misordered oplog errors.
Limit poolsize for "last oplog entry" to 1.

This ensures that we allocate sequencers in increasing order.
Add more info to the misordered oplog errors.
Add some facts to oplog.
wip
no, don't remove
typos
ooops
don't count the whole table.
dont update.
use less data
convert to chat.
dont send all rooms
lighten up a bit
only fetch rooms once.
more aggressive
Autoscale rooms. No fixed room count to move around by hand.
typo makes everything worse. also, index.
Add facts to chat-benchmark.