Hold recent messages in Aphlict so they can be replayed after clients reconnect

Summary:
Ref T12563. Before broadcasting messages from the server, store them in a history buffer.

A future change will let clients retrieve them.

Test Plan:

• Used the web frontend to look at the buffer, reloaded over time, sent messages. Saw buffer size go up as I sent messages and fall after 60 seconds.
• Set size to 4 messages, sent a bunch of messages, saw the buffer size max out at 4 messages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12563

Differential Revision: https://secure.phabricator.com/D17707