Add internal thread-safe dictionary wrapper

[nathandud]

To address issue #114, this PR includes an internal wrapper for Dictionary called CBMDictionary.

Some notes:

• The CBMDictionary does not attempt to include every possible dictionary operation - only what is needed by the current implementation of CoreBluetoothMock.
• The private queue uses the default quality-of-service. I believe this is sufficient.

Running the same unit test in #114 100,000 times repeatedly passes successfully:

Testing the performance of CBMCentralManagerMock.simulatePeripherals([device]) with the CBMDictionary was promising. In 100 tests, the average time:

• CBMDictionary: 46.71 microseconds
• Dictionary (Current): 46.04 microseconds

[CLAassistant]

All committers have signed the CLA.

[philips77]

Great job!

[philips77]

Hi, any objections of making CBMDictionary internal, or you'd prefer to expose it?

[nathandud]

@philips77 No objections on my end! I didn't add the internal keyword because it is internal by default but I see no issue with making it explicit.

[philips77]

Released in 1.0.0.