MINOR: spoe: Remove SPOE details from the appctx structure

Now, as for peers, we use an opaque pointer to store information related to the
SPOE filter in appctx structure. These information are now stored in a dedicated
structure (spoe_appctx) and allocated, using a pool, when the applet is created.

This removes the dependency between applets and the SPOE filter and avoids to
eventually inflate the appctx structure.
diff --git a/include/types/applet.h b/include/types/applet.h
index 851948b..90484f6 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -84,13 +84,7 @@
 			struct task *task;
 		} hlua_apphttp;                 /* used by the Lua HTTP services */
 		struct {
-			struct task *task;
-			void        *agent;
-			unsigned int version;
-			unsigned int max_frame_size;
-			unsigned int flags;
-			struct list  waiting_queue;
-			struct list  list;
+			void *ptr;              /* private pointer for SPOE filter */
 		} spoe;                         /* used by SPOE filter */
 		struct {
 			const char *msg;        /* pointer to a persistent message to be returned in CLI_ST_PRINT state */