Remove underscore from classes LBFactory_*, LoadBalancer_*, LoadMonitor_*

Part of program to remove underscores from class names. Checked core and
600+ extensions for occurrences. All uses are in core in core are updated
in this patch.

Patches for extensions:
* https://gerrit.wikimedia.org/r/#/c/96463/ CentralAuth
* https://gerrit.wikimedia.org/r/#/c/96460/ SimpleSecurity
* https://gerrit.wikimedia.org/r/#/c/96459/ WikiBase
* https://gerrit.wikimedia.org/r/#/c/96472/ Wikimedia configuration

Antoine added tests to test the back compatibility.

Change-Id: I507ba00a83bca8375a0215f00891b47e3c7afb9f