simulator: added TCP, general cleanup, lockstep

This adds the option to connect over UDP or TCP for mavlink to the
SITL simulator.

Also, this includes a bunch of general cleanup and refactoring of the
simulator interface code. For instance sending of mavlink messages was
put into separate functions and unneccessary comments were removed.

Also, this now sets the timestamp sent by the SITL simulator in the
HIL_SENSOR message in order to enable lockstep.