Revert "Refactored use of $wgMemc in JobQueueDB to use a field."

This reverts commit 7293699561f8751c66d55697e63b13a18653ac6b

Unit tests fail.

Change-Id: Iac8025844b37c584e451a4120e54898ae4f2524e
Merge "Revert "Refactored use of $wgMemc in JobQueueDB to use a field.""