Add some docstrings to driver base

Eliminates spurious missed lines from coverage report.
Don't raise a driver-specific error on send

Similar to doing listen() on the server side, if the driver throws an
exception when we do a cast() or call() we should translate it into
a transport-agnostic exception.
Add a client call timeout test
Remove a fixed fixme
Add docstrings for base executor class

Avoiding spurious missed lines in coverage report.
Call wait() in server tests

Just for the sake of the coverage report.
Fix thinko in exchange_from_url()

Funnily enough, this was just shown by the coverage report.
Remove unused statements in base serializer
Check start() does nothing on a running server