BUG/MEDIUM: spoe: Never create new spoe applet if there is no server up

BUG/MEDIUM: spoe: Never create new spoe applet if there is no server up

This test was already performed when a new message is queued into the
sending queue. However not when the last applet is released, in
spoe_release_appctx(). It is a quite old bug. It was introduced by commit
6f1296b5c7 ("BUG/MEDIUM: spoe: Create a SPOE applet if necessary when the
last one is released").

Because of this bug, new SPOE applets may be created and quickly released
because there is no server up, in loop and while there is at least one
message in the sending queue, consuming all the CPU. It is pretty visible if
the processing timeout is high.

To fix the bug, conditions to create or not a SPOE applet are now
centralized in spoe_create_appctx(). The test about the max connections per
second and about number of active servers are moved in this function.

This patch must be backported to all stable versions.