BUG/MAJOR: dns: Make the do-resolve action thread-safe

BUG/MAJOR: dns: Make the do-resolve action thread-safe

The do-resolve HTTP action, performing a DNS resolution of a sample expression
output, is not thread-safe at all. The resolver object used to do the resolution
must be locked when the action is executed or when the stream is released
because its curr or wait resolution lists and the requester list inside a
resolution are updated. It is also important to not wake up a released stream
(with a destroyed task).

Of course, because of this bug, various kind of crashes may be observed.

This patch should fix the issue #236. It must be backported as far as 2.0.

(cherry picked from commit 5098a08c2fafb0d9513996729d2a30c9785378f3)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 99f4623952cbbad2bcae451abdd0f3133bcbe75c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>