fdmap: prevent possible/theoretical race

With a GVL-free Ruby implementation, the following situation
may occur as FDs are recycled frequently with 3 threads
running:

expiry                 | normal    | acceptor
-----------------------+-----------+----------------------
yahns_expire(fd).enter |           |
                       | close(fd) |
                       |           | accept().return => fd
shutdown(fd) - WRONG   |           |
-----------------------+-----------+----------------------

So we must prevent expiry from running while another thread
is closing, otherwise there is a small chance a lack of
lock inside the Ruby implementation itself can lead to a
mis-issued close() to a fast-recycled FD.

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