* Added $wgCaptchaFileBackend configuration setting.
This causes captchas to be stored in the registered backend.
* Added $wgMemc usage to reduce directory operation I/O.
* Improved $wgCaptchaDeleteOnSolve in terms of race conditions.
* Added maintenance script to create and store new captchas based
on the total amount desired and the current number remaining.
Change-Id: Iac490503fb94a8f003e3b7e1474a949b037af5df
