BUG/MEDIUM: spoe: Kill applets if there are pending connections and nbthread > 1

BUG/MEDIUM: spoe: Kill applets if there are pending connections and nbthread > 1

When the processing stage is finished for a SPOE applet, before returning it
into the idle list, we check if the assigned server appears as full or if
there are some pending connections on the backend or the assigned server. If
yes, it means we reach a maxconn and we close the applet to free a
slot. Otherwise, the applet can be reused. This test is only performed if
there are more than one thread.

It is important to close SPOE applets when there are pending connections for
multithreaded instances because connections with the SPOE agents are
persistent and local to a thread (applets are local to a thread). If a
maxconn is configured, some threads may take all available slots for a
while, leaving remaining threads without any free slot to process SPOE
messages. It is especially true if the maxconn is low.

This patch should fix the issue #705. It must be backported as far as
1.8. However, the code in 1.8 is quite different, a test must be performed
to be sure it works well.