BUG/MEDIUM: spoe: Fix policy to close applets when SPOE connections are queued

BUG/MEDIUM: spoe: Fix policy to close applets when SPOE connections are queued

It is the second part of the fix that should solve fairness issues with the
connections management inside the SPOE filter. Indeed, in multithreaded
mode, when the SPOE detects there are some connections in queue on a server,
it closes existing connections by releasing SPOE applets. It is mandatory
when a maxconn is set because few connections on a thread may prenvent new
connections establishment.

The first attempt to fix this bug (9e647e5af "BUG/MEDIUM: spoe: Kill applets
if there are pending connections and nbthread > 1") introduced a bug. In
pipelining mode, SPOE applets might be closed while some frames are pending
for the ACK reply. To fix the bug, in the processing stage, if there are
some connections in queue, only truly idle applets may process pending
requests. In this case, only one request at a time is processed. And at the
end of the processing stage, only truly idle applets may be released. It is
an empirical workaround, but it should be good enough to solve contention
issues when a low maxconn is set.

This patch should partely fix the issue #1340. It must be backported as far
as 2.0.