Allow only one download for a given image at a time, so that we don't end up downloading the same image twice if two clients request a remote image at the same time

Limit the size of images that are thumbnailed serverside. Limit the size of file that a server will download from a remote server

Federation retries

turn back on per-request transaction retries, so that every time we try to hit a dead server we actually end up hammering 5 times :|

add errbacks to enqueue_pdu deferreds; change logging for failed federation sends to warn rather than exception

track replication destination health, and perform exponential back-off when sending transactions. does *not* yet retry transactions, but drops them on the floor if waiting for a server to recover.

Store any incoming presence push in the local cache anyway, even if there's no interested observers (yet *hint*) (SYN-115)