R1:0ad2b526bcf4

Don't mention the same object in IRC more than once every 10 minutes

Summary:
phabot gets spammy when discussing a specific revision.

Test Plan:
Added debugging code and produced a reasonable-looking chatlog:

[1:15pm] phabotlocal joined the chat room.
[1:15pm] epriestley: D5
[1:15pm] phabotlocal: D5 quack - http://local.aphront.com/D5
[1:15pm] epriestley: D5
[1:15pm] phabotlocal: Declining to mention PHID-DREV-d7940d0fe081e3ae5267
again (59s left).
[1:15pm] epriestley: D5
[1:15pm]…