[ansd]

Follows through on the decision in #16077 (comment) to remove parsing of the AMQP 1.0
compatibility headers in 4.4 rather than harden their validation.

RabbitMQ 3.13 and earlier stored a message received over AMQP 1.0 as an
AMQP 0-9-1 #content{}, keeping the sections that do not map to AMQP 0-9-1 in
four headers:

x-amqp-1.0-properties
x-amqp-1.0-app-properties
x-amqp-1.0-message-annotations
x-amqp-1.0-footer

The module that wrote them was replaced in 4.0, so the only messages that carry
them are ones a 3.13 or earlier node stored and no consumer has read since.
mc_amqpl still looked all four up on every conversion to AMQP 1.0. Such a
message is still delivered, with these headers now converted like any other
x- header, which is what an AMQP 0-9-1 consumer of one has always seen.

Three message annotations have the same history in the other direction:
x-basic-delivery-mode, x-basic-priority and x-basic-expiration.
rabbit_msg_record wrote them when it encoded an AMQP 0-9-1 message as AMQP 1.0
for a stream, and it was deleted before 4.0, so only a stream entry written by a
3.13 or earlier node carries them. x-basic-type stays, because
basic.properties.type has no AMQP 1.0 equivalent.

Two smaller changes to the same conversion code:

• basic.properties.type is a free-form AMQP 0-9-1 property, but convert_to/3
  took a value of amqp-1.0 to mean the payload holds AMQP 1.0 encoded body
  sections. It now checks that the payload is well formed and that the sections
  it decodes to belong in a message body, and falls back to a single data
  section otherwise.

• essential_properties/1 matched {ok, TTL} on
  rabbit_basic:parse_expiration/1. rabbit_channel checks expiration before
  building the message, but producers that build a #content{} themselves do
  not go through that check, so an expiration that does not parse is now
  ignored. This changes nothing for AMQP 0-9-1.

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[michaelklishin]

Should we apply the same idea as in 7111f4d to mc_amqpl:convert_to/3 (lines 341-347)?

Something like this

MsgTTL = case rabbit_basic:parse_expiration(Props) of
             {ok, Ttl} -> Ttl;
             {error, _} -> undefined
         end,