MDL-25500 Generic plugin based locking framework for cron (and more).
MDL-25500 locking: Added mysql locks.
MDL-25500: Fix undefined variable warning
MDL-25500: Added a generic db locktype and help improvements.

Moved the admin description of each plugin into the plugins lang file.
MDL-25502: Added "black magic" task allocator for cron
MDL-22502: Change the task rescheduler to schedule retries for failed jobs.

Failed tasks will be rescheduled initially 1 minute later, then 2 minutes, then 4 minutes etc.
The longest a task will have to wait after being rescheduled for failing is 24 hours.
MDL-25503: Add step to cron to run all scheduled tasks
MDL-25504: Add/remove scheduled tasks on install
MDL-25504: Convert session_gc to scheduled task.
MDL-25504 cron: Move delete unconfirmed users to task
MDL-25504 cron: Improve cron task mtrace format

Remove extra newlines.
MDL-25504 cron: Convert delete incomplete users to task
MDL-25504 cron: Convert delete old logs to task.
MDL-25504 cron: cleanup backup tables converted to task
MDL-25504 cron: Convert delete cache text to task
MDL-25504 cron: Convert tag cron to task
MDL-25504 cron: Convert context cleanup to task
MDL-25504 cron: Convert cache cleanup to task
MDL-25504 cron: convert messaging cleanup to task
MDL-25504: Align some cron printouts