MDL-41245 fix redirect and missing string when cannot automatically delete uninstalled plugin
Merge branch 'w35_MDL-41245_m26_pluginuninstall' of https://github.com/skodak/moodle
MDL-25500 lock: New pluggable locking framework.
MDL-25500 locking: Only allow one memcache lock server.

Failover does not make sense for locks because the memcache servers
may not replicate the locks.
Also cleaned up config-dist comments and remove overly escaped slashes.
MDL-25500 locking: Fix a range of issues found in peer review

Specifically:
* In db lock - fix phpdocs
* In file lock, set the lockfile back to null, instead of 0
* Renamed is_... functions to supports_ for clarity
* Add a separator to keys for memcache locks
* In memcache return result of delete
* Remove unused code in db lock
* Change locking loops to do/while
* Add spacing around => operator everywhere