MINOR: mux-h2: add a new setting, "tune.h2.log-errors" to tweak error logging

MINOR: mux-h2: add a new setting, "tune.h2.log-errors" to tweak error logging

The H2 mux currently logs whenever some decoding fails. Most of the errors
happen at the connection level, but some are even at the stream level,
meaning that multiple logs can be emitted for a given connection, which
can quickly use some resource for little value. This new setting allows
to tweak this and decide to only log errors that affect the connection,
or even none at all.

This should be backported at least as far as 3.2.