IOVSvc: Remove locking from CondSvc::isValidID.

The locking within CondSvc::isValidID is heavily contended in jobs with
many threads (32-64).  Rework so that we don't need to use locking.

Rather than doing a SG lookup each time isValidID is called, we instead
build a map from SG key to CondCont instances at the end of initialization.
(This is called from CondInputLoader::start).  We can then look up the
key directly in this map rather then querying SG.  Since this map
is modified only during START, we don't need any locking.