Implement smart waits for rarely updated repositories

Summary:
Ref T4605. When figuring out how long to wait to update a repository, factor in when it was last pushed. For rarely updated repositories, wait longer between updates.

(A slightly funky thing about this is that empty repos update every 15 seconds, but that seems OK for the moment.)

Test Plan:
Ran `bin/phd debug pulllocal` and saw sensible calculations and output:

```
...
<VERB> PhabricatorRepositoryPullLocalDaemon Last commit to repository "rPOEMS" was 1,239,608 seconds ago; considering a wait of 6,198 seconds before update.
>>> [79] <query> SELECT * FROM `repository` r   ORDER BY r.id DESC
<<< [79] <query> 514 us
>>> [80] <query> SELECT * FROM `repository_statusmessage` WHERE statusType = 'needs-update'
<<< [80] <query> 406 us
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rINIH" is not due for an update for 8,754 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rDUCK" is not due for an update for 14 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rMTESTX" is not due for an update for 21,598 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rQWER" is not due for an update for 14 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rBT" is not due for an update for 13 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rSVNX" is not due for an update for 21,598 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rINIG" is not due for an update for 13 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rHGTEST" is not due for an update for 21,598 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rBTX" is not due for an update for 14 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rGX" is not due for an update for 13 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rMTX" is currently updating.
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rPOEMS" is not due for an update for 6,198 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rPHU" is currently updating.
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rSVN" is not due for an update for 21,598 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rPHY" is currently updating.
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rGTEST" is not due for an update for 21,598 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rINIS" is not due for an update for 6,894 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rARCLINT" is not due for an update for 21,599 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rLPHX" is not due for an update for 1,979 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rARC" is not due for an update for 1,824 second(s).
<VERB> PhabricatorRepositoryPullLocalDaemon Repository "rINIHG" is not due for an update for 21,599 second(s).
...
```

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4605

Differential Revision: https://secure.phabricator.com/D8782