Refactor timers

[jthomas43]

This PR replaces 5 libuv timers with 1 libuv timer, based on the observation that only one of these timers need to be active at a time:

1. Retransmission timer (RTO)
2. Tail-Loss Probe timer (TLP)
3. Rack-Reorder timer (RACK_REO)
4. Zero-Window probe timer (ZWP)
5. KeepAlive probe timer (KeepAlive)

(4) Zero-Window probe and (5) KeepAlive can only be active if there are no packets in flight, ie all data has been acked. If the window size is zero then the ZWP timer is set instead of KeepAlive

(1) RTO, (2) TLP or (3) RACK_REO may be active when data is in flight.. The TLP is set when no loss is detected and the RACK_REO is set when potential loss (in the form of a SACK) is received, both of these timers reset the RTO timer on expiration.