reuse composed messages

[slingamn]

The devel+sharedbuffers branch is a prototype of this; IIRC the speedup on the chanflood benchmark was about 30%. We'll probably get a bit less.

Here's the proposal: for each PRIVMSG/NOTICE/TAGMSG, cache 2 versions of the built message. One is for clients with message-tags negotiated: these clients are eligible to receive account-tag (where applicable), server-time, etc., so we'll send them a cached "full" message with all tags. The other is for clients that have no tag capabilities negotiated (neither message-tags, nor server-time, nor account-tag, nor anything else that would indicate the ability to receive tags): they'll get a cached message with no tags. Other clients will go through the current flow where we build an individual copy of the message.