Simplify thread spawning

The `isc_trampoline` module had a lot of machinery to support stable
thread IDs for use by hazard pointers. But the hazard pointer code
is gone, and the `isc_loop` module now has its own per-loop thread
IDs.

The trampoline machinery seems over-complicated for its remaining
tasks, so move the per-thread initialization into `isc/thread.c`,
and delete the rest.

lib/isc/include/isc/thread.h

@@ -30,8 +30,6 @@
 #include <isc/lang.h>
 #include <isc/result.h>
 
-extern thread_local size_t isc_tid_v;
-
 ISC_LANG_BEGINDECLS
 
 typedef pthread_t isc_thread_t;