dnstap: Add 'log-queries' and 'log-responses' module options

This commit adds two new boolean options to the dnstap module
configuration.

"log-queries" and "log-responses" independently control whether query
messages and response messages will be logged, respectively. They both
default to "on", which matches the behavior of previous versions of
Knot.

This provides similar functionality to BIND and Unbound, whose dnstap
implementations offer control over which types of messages are logged by
dnstap.