Rename allocator to identifierAllocator and add an identifierMutex to make...

Rename allocator to identifierAllocator and add an identifierMutex to make identifier uniquing thread safe. This also adds a general purpose 'contextMutex' to protect access to the rest of the miscellaneous parts of the MLIRContext, e.g. diagnostics, dialect registration, etc. This is step 5/5 of making the MLIRContext thread-safe.

PiperOrigin-RevId: 238516697