Implement attach for debug session.

This commit keeps in place the now-empty SlaveSessionImpl. If it ends up
being purely useless, I'll get rid of it later when I restart working on
platform support.
Ignore packet checksums when we are in noAckMode.

noAckMode implies that checksums should not be checked. Implementations
may simply send "#00" as their checksum.
Separate the run and wait operations in ProcessSpawner.