Add version string to initial debug message

Add version string to initial debug message
Work out when the clock can be relied upon not to go to sleep, i.e. is "active". This is when SPS is playing (B) or paused (P) and for brief periods after the clock is selected (T) and after play ends (E).
SPS signals Play/Pause/End/Timer Select/Timer Release with B/P/E/T messages -- B for "Begin". "T <ip>" means select the clock at that address. "T" means release the clock.
Reset clock smoothing when the clock goes "active" but not at any other time.
Clamp reductions in the offset calculated as a uint64_t remote-time-now_ns - local-time-now_ns -- and which it is assumed are due to delays in the network when the clock is "active" -- to 2.5 mS.
Turn off clock updates when the clock is not "active" -- i.e. when SPS is not paying or paused.
Remove some redundant features and code.