BUG-5280: separate request sequence and transmit sequence

BUG-5280: separate request sequence and transmit sequence

Clean up the confusion in sequence numbers. There are actually
two sequences:
- logical request sequence, which indicates the order of requests
  in which they should be applied to the target entity. It is
  assigned by logic emitting the request.
- transmit sequence within a connection to the backend, as
  initiated by ConnectClientRequest. It is assigned by SequencedQueue
  as it is transmitting requests and reset when a new connection
  to the backend is made.

This requires establishing the concept of a session, which is
a single conversation between frontend and backend. It is severed
whenever the frontend times out and re-established when the leader
is found and it responds with ConnectClientSuccess.

The sending of ConnectClientRequest is not done via the queue,
as it is part of backend resolution process. Since this is not
a performance-critical path, we use simple Patterns.ask() to
send the request and get completion notification -- which we then
translate to ShardBackendInfo.

ConnectClientSuccess gives us backend-preferred version and
backend-specified cap on the number of outstanding messages then
it can handle concurrently. This maximum is used to limit the
transmit path of SequencedQueue, so that it does not attempt
to send more requests at any given time.

Internal queue for unsent requests is kept unbounded for now,
subject to a Semaphore-driven throttle in a follow-up patch.

Change-Id: I61663073bf6632c1ed8c036dee37f1ac39cf7794
Signed-off-by: Robert Varga <rovarga@cisco.com>