Introduce a generic lws_state object with notification handlers
that may be registered in a chain.
Implement one of those in the context to manage the "system state".
Allow other pieces of lws and user code to register notification
handlers on a context list. Handlers can object to or take over
responsibility to move forward and retry system state changes if
they know that some dependent action must succeed first.
For example if the system time is invalid, we cannot move on to
a state where anything can do tls until that has been corrected.
Refactor everything around ping / pong handling in ws and h2, so there
is instead a protocol-independent validity lws_sul tracking how long it
has been since the last exchange that confirms the operation of the
network connection in both directions.
Clean out periodic role callback and replace the last two role users
with discrete lws_sul for each pt.
Add a generic table-based backoff scheme and a helper to track the
try count and calculate the next delay in ms.
Allow lws_sequencer_t to be given one of these at creation time...
since the number of creation args is getting a bit too much
convert that to an info struct at the same time.
