MINOR: thread: use a dedicated static pthread_t array in thread.c

MINOR: thread: use a dedicated static pthread_t array in thread.c

This removes the thread identifiers from struct thread_info and moves
them only in static array in thread.c since it's now the only file that
needs to touch it. It's also the only file that needs to include
pthread.h, beyond haproxy.c which needs it to start the poll loop. As
a result, much less system includes are needed and the LoC reduced by
around 3%.