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.
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.