[9.20] fix: dev: Fix data race during rndc dumpdb or zone load

'rndc dumpdb' against a server with zones, and async zone load,
had a timing window where the operation's completion could fire
before the server had finished registering the operation,
occasionally leading to a possible crash.  The completion is now
delivered after the registration is in place.

Closes #5952

Backport of MR !11991

Merge branch 'backport-5952-fix-masterdump-async-ctx-race-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12021