* anders/diameter/distribution/OTP-15398:
  Add diameter_dist_SUITE to exercise diameter_dist:route_session/2
  Add consistent hashing to diameter_dist:route_session/2
  Add options to diameter_dist:route_session/2 node selection
  Add diameter_dist for ready spawn_opt callbacks
  Tweak/document request handler callback
  Document acknowledgements in transport interface
  Fix comment typo

The possibility of configuring an MFA as spawn_opt was added in commit
fd285079, the callback being passed an arity-0 fun to be applied in an
appropriate handler process. Replace the fun by a tuple to be passed to
diameter_traffic:request/1, to avoid passing funs between nodes when
handler processes are remote.

A list-valued spawn_opt is now equivalent to the following configured as
{spawn_opt, {Mod, spawn_local, [Opts]}}.

  spawn_local(ReqT, Opts) ->
      spawn_opt(diameter_traffic, request, [ReqT], Opts).

ReqT is passed by diameter and contains information that the callback
may want to decide where to handle the request in question (which wasn't
accessible with a fun), but this information isn't exposed in a
documented way. The intention is instead to add an own callback
implementation to make use of the information.

Note that application lookup now takes place in the watchdog process in
both the list-valued (or no configuration) and mfa-valued cases. Whether
this is good, bad, or (probably) inconsequential remains to be seen.

This is the {diameter, ack} message from a transport process to its
parent that requests that every Diameter request passed to the parent be
matched by a incoming send message, the atom false replacing a message()
in the case that the request is unanswered, allowing the transport to
keep track of the number of outstanding requests.

These were added in commit ca09cf7b, and are used to implement the
message_cb config in diameter_tcp/sctp, documented in commit cefcaa5c.

This commit documents the interface, to make it available to other
transport implementations.

* maint-20:
  Updated OTP version
  Prepare release

introduced after OTP_R13B03.