Refactored messenger replacement after identifying an inbound TCP con…

…nection.

Added back in a line that stores the TCP socket after establishing a …

…connection; deleting it was a mistake.

Also, ramped down the logging level in unit tests, as had accidentally checked in a more verbose setting.

Added more tests for concurrent RPCs, expanded transport documentatio…

…n to provide warnings on its use, and exposed Request/Response types used in RPC.