fix: dev: Fix datarace between unlocking fctx lock and shuttingdown fctx

There was a data race where new fetch response could be added to the
fetch context after we unlock the fetch context and before we shut it
down.  This could cause assertion failure when fctx__done() was called
with ISC_R_SUCCESS because there was originally no fetch response, but
new fetch response without associated dataset was added before we had a
chance to shutdown the fetch context.  This manifested in the
validated() callback, where cache_rrset() now returns ISC_R_SUCCESS
instead of DNS_R_UNCHANGED when cache was not changed.  However the data
race was wrong on a general level.

Add new argument to fctx__done() that allows to call it with fctx->lock
already acquired to prevent these data races.

Closes #5507

Merge branch '5507-dont-release-fctx-lock-on-done' into 'main'

See merge request isc-projects/bind9!10961