BUG/MINOR: action: do-resolve does not yield on requests with body

BUG/MINOR: action: do-resolve does not yield on requests with body

@davidmogar reported a github issue (#227) about problems with
do-resolve action when the request contains a body.
The variable was never populated in such case, despite tcpdump shows a
valid DNS response coming back.

The do-resolve action is a task in HAProxy and so it's waken by the
scheduler each time the scheduler think such task may have some work to
do.
When a simple HTTP request is sent, then the task is called, it sends
the DNS request, then the scheduler will wake up the task again later
once the DNS response is there.
Now, when the client send a PUT or a POST request (or any other type)
with a BODY, then the do-resolve action if first waken up once the
headers are processed. It sends the DNS request. Then, when the bytes
for the body are processed by HAProxy AND the DNS response has not yet
been received, then the action simply terminates and cleans up all the
data associated to this resolution...

This patch detect such behavior and if the action is now waken up while
a DNS resolution is in RUNNING state, then the action will tell the
scheduler to wake it up again later.

Backport status: 2.0 and above