The snapshot sent right after SUBSCRIBE was taken from the previous
frame, which on the first frame of a session meant every value came back
as UNAVAILABLE. The link now publishes the state of the current frame
before the received frames are processed.
Adds the logical protocol layer on top of the frame codec: session
handshake with negotiated frame size, session scoped symbol handles,
state subscriptions with periodic, on change and deadband filtering,
command events with duplicate protection and continuous controls with
stale update rejection, plus per connection rate limits and diagnostics.
A single worker thread owns all of the blocking transport work and
exchanges finished frames with the simulation thread through short locked
queues, so a stalled, flooding or unplugged controller cannot hold up the
simulation.
