Moved job running via $wgJobRunRate to a special API

Moved job running via $wgJobRunRate to a special API

* A background internal HTTP request used to load the API.
  This handles job execution and related periodic tasks.
  It avoids blocking the main request or breaking it on fatals.
* This method avoids problems that shelling could have:
** Mismatched apache/cli PHP config or versions.
** Had to start a new process and could not use opcode cache.
** safe_mode and open_basedir restrictions required a fallback.
** Some wiki farms may not respect the --wiki parameter.
** wgMaxShellMemory applies to spawned PHP processes.
** Spawning processes is more prone to DOS due to a lack
   of proper limiting/pooling.

bug: 60208
bug: 60210
bug: 60698
bug: 60844
Change-Id: I78b0e709301ac4a0c7b7ed337d6969d7546674bf