Create skeleton for PinnedConnectionTaskExecutor

Create a skeleton for a new type of TaskExecutor that uses a single transport connection for all RPC scheduled through it. This task executor should own another TaskExecutor and implement most of the TaskExecutor API by dispatching to the owned TaskExecutor. It should leave the RPC portions of the API (scheduleRemoteCommandOnAny) unimplemented to be filled out later. The eventual implementation will acquire a unique connection from the NetworkInterface owned by this task executor when the first RPC is scheduled, and use that connection for all future RPCs for the lifetime of the PinnedConnectionTaskExecutor.