MINOR: threads: serialize threads initialization

MINOR: threads: serialize threads initialization

There is no point in initializing threads in parallel when we know that
it's the moment where some global variables are turned to thread-local
ones, and/or that some global variables are updated (like global_now or
trash_size). Some FDs might be created/destroyed/reallocated and could
be tricky to follow as well (think about epoll_fd for example).

Instead of having to be extremely careful about all these, and to trigger
false positives in thread sanitizers, let's simply initialize one thread
at a time. The init step is very fast so nobody should even notice, and
we won't have any more doubts about what might have happened when
analysing a dump.

See GH issues #111 and #117 for some background on this.