When we fail to acquire a repository lock, try to provide a hint about why

Summary:
Ref T13202. See PHI889. If the lock log is enabled, we can try to offer more details about lock holders.

When we fail to acquire a lock:

• check for recent acquisitions and suggest that this is a bottleneck issue;
• if there are no recent acquisitions, check for the last acquisition and print details about it (what process, how long ago, whether or not we believe it was released).

Test Plan:

• Enabled the lock log.
• Changed the lock wait time to 1 second.
• Added a sleep(10) after grabbing the lock.
• In one window, ran a Conduit call or a git fetch.
• In another window, ran another operation.
• Got useful/sensible errors for both ssh and web lock holders, for example:

PhutilProxyException: Failed to acquire read lock after waiting 1 second(s). You may be able to retry later. (This lock was most recently acquired by a process (pid=12609, host=orbital-3.local, sapi=apache2handler, controller=PhabricatorConduitAPIController, method=diffusion.rawdiffquery) 3 second(s) ago. There is no record of this lock being released.)

PhutilProxyException: Failed to acquire read lock after waiting 1 second(s). You may be able to retry later. (This lock was most recently acquired by a process (pid=65251, host=orbital-3.local, sapi=cli, argv=/Users/epriestley/dev/core/lib/phabricator/bin/ssh-exec --phabricator-ssh-device local.phacility.net --phabricator-ssh-key 2) 2 second(s) ago. There is no record of this lock being released.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13202

Differential Revision: https://secure.phabricator.com/D19702