Fix a potential tsd cleanup leak.

Prior to 767d8506 (Refactor arenas array
(fixes deadlock).), it was possible under some circumstances for
arena_get() to trigger recreation of the arenas cache during tsd
cleanup, and the arenas cache would then be leaked.  In principle a
similar issue could still occur as a side effect of decay-based purging,
which calls arena_tdata_get().  Fix arenas_tdata_cleanup() by setting
tsd->arenas_tdata_bypass to true, so that arena_tdata_get() will
gracefully fail (an expected behavior) rather than recreating
tsd->arena_tdata.

Reported by Christopher Ferris <cferris@google.com>.