MEDIUM: backend: use a trylock to grab a connection on high FD counts as well

MEDIUM: backend: use a trylock to grab a connection on high FD counts as well

Commit b1adf03df ("MEDIUM: backend: use a trylock when trying to grab an
idle connection") solved a contention issue on the backend under normal
condition, but there is another one further, which only happens when the
number of FDs in use is considered too high, and which obviously causes
random crashes with just 16 threads once the number of FDs is about to be
exhausted.

Like the aforementioned patch, this one should be backported to 2.3.

(cherry picked from commit 9b9f8477f8c751e366a526e2177a8aab34c80f6d)
[backported for the same reason as the first one above -- managed to
 crash on a 8c16t Xeon without it; minor ctx adjustments (list vs tree)
 and lock name]
Signed-off-by: Willy Tarreau <w@1wt.eu>