Feature: add log_backend_messages.

Feature: add log_backend_messages.

When enabled, log protocol messages from each backend. Possible
options are "none", "terse" and "verbose". "none" disables the feature
and is the default. "verbose" prints the log each time pgpool receives
a message from backend. "terse" is similar to verbose except it does
not print logs for repeated message to save log lines. If different
kind of message received, pgpool prints a log message including the
number of the message. One downside of "terse" is, the repeated
message will not be printed if the pgpool child process is killed
before different kind of message arrives.

For testing, 039.log_backend_messages is added.

Discussion: [pgpool-hackers: 4535] New feature: log_backend_messages
https://www.pgpool.net/pipermail/pgpool-hackers/2024-November/004536.html