Add lock_guard to thread_pool::thread_count

I think there's a possible race when calling thread_pool::thread_count
from a worker thread, if the main thread changes the number of threads
at the same time.

When this code was initially written, we didn't worry about this,
because this was only accessed from the main thread.  This is no
longer the case, though.

This patch fixes any potential problem by adding a lock_guard to the
method.  This doesn't seem too harmful because this is not called very
much and because I doubt this lock is highly contended.

While doing this I noticed that thread_pool::do_post_task could access
m_sized_at_least_once and m_thread_count without holding the lock.
This patch changes this method as well.

Approved-by: Kevin Buettner <kevinb@redhat.com>