ssl: support multi-frag certificate assembly

ssl: support multi-frag certificate assembly

Support reassembling multi-frag certificates. For this the cert queuing
code is changed to queue just the cert, not entire tls record.

Improve message tracking. Better track where a message starts and ends
before passing data around.

Add wrapper macros to check for 'impossible' conditions that are activate
in debug validation mode. This helps fuzzers find input that might trigger
these conditions, if they exist.