Use thread safe collection for static in L1MuGMTReadoutCollection

L1MuGMTReadoutCollection::getRecord returns a const reference to a
dummy item if the requested item is not in its main collection. Prevously
this was done by using a static std::map to hold any requested missing
items. To make this thread safe we changed to a tbb::concurrent_unordered_map.
Merge pull request #1320 from Dr15Jones/threadSafeFixesDataFormatsL1GlobalMuonTrigger

Multithreading fixes -- Use thread safe collection for static in L1MuGMTReadoutCollection