Improve PQtrace() output format

Improve PQtrace() output format

Transform the PQtrace output format from its ancient (and mostly
useless) byte-level output format to a logical-message-level output,
making it much more usable.  This implementation allows the printing
code to be written (as it indeed was) by looking at the protocol
documentation, which gives more confidence that the three (docs, trace
code and actual code) actually match.

Author: 岩田 彩 (Aya Iwata) <[email protected]>
Reviewed-by: 綱川 貴之 (Takayuki Tsunakawa) <[email protected]>
Reviewed-by: Kirk Jamison <[email protected]>
Reviewed-by: Kyotaro Horiguchi <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Reviewed-by: 黒田 隼人 (Hayato Kuroda) <[email protected]>
Reviewed-by: "Nagaura, Ryohei" <[email protected]>
Reviewed-by: Ryo Matsumura <[email protected]>
Reviewed-by: Greg Nancarrow <[email protected]>
Reviewed-by: Jim Doty <[email protected]>
Reviewed-by: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/71E660EB361DF14299875B198D4CE5423DE3FBA4@g01jpexmbkw25