Refs #3204, added extra info message to privacy manager purge function when lock tables privilege is not granted to user & replaced lock tables sql in ArchiveProcessing w/ GET_LOCK.

Notes:

  * Added Piwik_GetDbLock & Piwik_ReleaseDbLock functions.



git-svn-id: http://dev.piwik.org/svn/trunk@6485 59fd770c-687e-43c8-a1e3-f5a4ff64c105