chg: dev: Change isc_tid to be isc_tid_t type (a signed integer type)

Change the internal type used for isc_tid unit to isc_tid_t to hide the
specific integer type being used for the 'tid'.  Internally, the isc_tid
unit is now using signed integer type.  This allows us to have negatively
indexed arrays that works both for threads with assigned tid and the
threads with unassigned tid.  Additionally, limit the number of threads
(loops) to 512 (compile time default).

Merge branch 'ondrej/tidy-up-isc-tid' into 'main'

See merge request isc-projects/bind9!10656