Draft: Attempt to allow more threads used in hp

Petr Menšík requested to merge pemensik/bind9:rhbz/fedora/2048235-v9_16 into v9_16

bind-dyndb-ldap uses additional threads, which fail on isc_hp_protect with tid higher than initialized in hp_init(). Initialize double of original hp pointers, but prepare only the same number of thread structures. Additional threads can be initialized by isc_hp_thread_check() function called from possible custom threads made on top.

Related to crash introduced by 9.16.25 release in bind-dyndb-ldap: https://bugzilla.redhat.com/show_bug.cgi?id=2048235

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