Avoid some possible deadlocks on account creation

Avoid some possible deadlocks on account creation

* This uses a non-blocking $wgMemc lock to reserve the user
  name in question. This should prevent two threads from
  reaching LOCK IN SHARE MODE and getting stuck on INSERT.
  The lock is global to better cover auth plugins.
* This adds a BagOStuff::getScopedLock() convenience method.
  It uses less queries than LockManager by being EX only.
* Avoid extra lock attempt in lock() in non-blocking mode.
* Removed (un)lock() HashBagOStuff overrides that made it
  behave differently than other caches (wrt to re-entrance).

Bug: T106850
Change-Id: Iecf95206d712367f5d202f76ab0eaa9d7bdabf2b