Refactor how ADB names and entries are stored in the dns_adb

Refactor how ADB names and entries are stored in the dns_adb

Replace adb->{names,entries} and related arrays (indexed by hashed
bucket) with a isc_ht hash tables storing the new struct
adb{name,entry}bucket_t that wraps all the variables that were
originally stored in arrays indexed by "bucket" number stored directly
in the struct dns_adb.

Previously, the task exclusive mode has been used to grow the internal
arrays used to store the named and entries objects.  The isc_ht hash
tables are now protected by the isc_rwlock instead and thus the usage of
the task exclusive mode has been removed from the dns_adb.

Co-authored-by: Ondřej Surý <ondrej@isc.org>