Fix list-valued Vendor-Specific-Application-Id config

Commit f0a36c79 broke the handling of such configuration, resulting in a
function clause error in diameter_capx if the list was not of length 3,
and faulty extraction of application id's otherwise.  Only record-valued
config was properly interpreted.
Prepare release
Comment tweaks
Ensure DWR isn't sent immediately after DWA

Having the peer_fsm process answer DWR meant that watchdog timer expiry
could result in an outgoing DWR despite the fact that an incoming DWR
was just answered. Having the watchdog process answer avoids this.

diameter_peer_fsm must be loaded before diameter_watchdog.  It's
possible for one incoming DWR to go unanswered but a subsequent DWR will
be answered so no harm is done.
Fix UTF8String encode

To accept any nested list of codepoints and binaries. A list containing
a binary was previously misinterpreted and the documentation was
incomplete.

Also, rework codec suite slightly to be able to specify values for which
decode o encode is the identity map, for which encode should succeed,
and for which encode should fail.
Use module unicode for UTF8String encode/decode

The original code predates that module but there's no reason not to use
it now.
Simplify Address encode/decode

By using binary comprehensions. Add string-valued addresses to codec
suite.