Simplify thread spawning
The isc_trampoline
module had a lot of machintery 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.