[9.20] new: usr: Add an rndc command to reset some statistics counters

The new ``reset-stats`` command for ``rndc`` allows some statistics
counters to be reset during runtime. At the moment only two "high-water"
counters are supported, so the ability to reset them after the
initial peaks during the server's "warm-up" phase may be useful for
some operators.

Closes #5251

Backport of MR !10318

Merge branch 'backport-5251-feature-rndc-reset-high-water-statistics-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10335