The `m_should_block` member variable that many of the Thread::Blocker
subclasses had was really only used to carry state from the constructor
to the immediate-unblock-without-blocking escape hatch.
This patch refactors the blockers so that we don't need to hold on
to this flag after setup_blocker(), and instead the return value from
setup_blocker() is the authority on whether the unblock conditions
are already met.
0c1d41cc8a