MINOR: spoe: Support the async mode with several threads

A different engine-id is now generated for each thread. So, it is possible to
enable the async mode with several threads.

This patch may be backported to older versions.
diff --git a/include/types/spoe.h b/include/types/spoe.h
index 9bba492..2dbf6e5 100644
--- a/include/types/spoe.h
+++ b/include/types/spoe.h
@@ -244,7 +244,6 @@
 	} timeout;
 
 	/* Config info */
-	char                 *engine_id;      /* engine-id string */
 	char                 *var_pfx;        /* Prefix used for vars set by the agent */
 	char                 *var_on_error;   /* Variable to set when an error occurred, in the TXN scope */
 	char                 *var_t_process;  /* Variable to set to report the processing time of the last event/group, in the TXN scope */
@@ -264,6 +263,7 @@
 
 	/* running info */
 	struct {
+		char           *engine_id;      /* engine-id string */
 		unsigned int    frame_size;     /* current maximum frame size, only used to encode messages */
 		unsigned int    processing;
 		struct freq_ctr processing_per_sec;