If VRT_AddDirector() was called from handling a VCL_COLD event or,
indirectly, from another thread which the VCL_COLD event handler was
waiting for, varnishd would deadlock and prevent any CLI or director
changes, because VRT_AddDirector() requires the vcl_mtx, which is held
during vcl_BackendEvent() to ensure a consistent view of the director
list. Because of the early return from VRT_AddDirector() this likely
only happened in VTC mode, but the underlying race existed
nevertheless.

This patch _almost_ fixes the issue with the intend of making it
highly unlikely to occur without getting too involved with the vcl
temperature controls: We now check the same conditions under which
vcl_set_state() would transition the temperature to COOLING and, if
they apply, use Lck_Trylock() in a try/wait loop instead of
Lck_Lock(), avoiding the deadlock.

The patch presumably still does not fix the problem entirely, because
the reads of vcl->busy and vcl->temp before the Lck_Trylock() could
still be outdated. With the temperature controls otherwise unchanged,
the only alternative idea I could come up with was to always use a
try/wait loop, which I dismissed due to the performance impact
(overhead and added latency).

Ref https://github.com/nigoroll/libvmod-dynamic/issues/110