ZMGR: TLS contexts cache - properly synchronise access

ZMGR: TLS contexts cache - properly synchronise access

This commit ensures that access to the TLS context cache within zone
manager is properly synchronised.

Previously there was a possibility for it to get unexpectedly
NULLified for a brief moment by a call to
dns_zonemgr_set_tlsctx_cache() from one thread, while being accessed
from another (e.g. from got_transfer_quota()). This behaviour could
lead to server abort()ing on configuration reload (under very rare
circumstances).

That behaviour has been fixed.