Incorporate changes from Cruise LLC

[dallison]

This incorporates changes from Cruise LLC. There are many changes but mostly enhancements to the APIs.

There are also a couple of functionality changes:

1. Message checksums: options on publisher and subscriber to produce and check a CRC32 on the message contents. Produced when message is published and checked when received by a subscriber. You can also add a custom checksum callback that will be called when a checksum is needed. This can be used if you have external checksum calculators (GPU for example).
2. Server plugins: allows functionality to be added to the server from a dynamically loaded module. You can do things like create additional channels and perform other actions when the server receives a request from a client.
3. Thread safety: optionally adds a lock on the client to allow the client to be used by multiple threads at once.
4. Portability changes including POSIX shared memory support for non-Linux systems.
5. Eliminated any memory allocation on the subscriber's ReadMessage path. There were a few minor things allocated by libraries and the ActiveMessage was allocated as a std::shared_ptr.
6. Add some more latency tests