Make Dispatch use VSL_Check to handle SHM ptrs.

This allows Dispatch to run completely in SHM with no copying for most
cases, also when transactions come in multiple batches.

include/vapi/vsl_int.h

@@ -89,6 +89,8 @@ struct VSL_head {
 #define VSL_BACKEND(ptr)	(((ptr)[1]) & VSL_BACKENDMARKER)
 #define VSL_DATA(ptr)		((char*)((ptr)+2))
 #define VSL_CDATA(ptr)		((const char*)((ptr)+2))
+#define VSL_BATCHLEN(ptr)	((ptr)[1])
+#define VSL_BATCHID(ptr)	(VSL_ID((ptr) + 2))
 
 #define VSL_ENDMARKER	(((uint32_t)SLT__Reserved << 24) | 0x454545) /* "EEE" */
 #define VSL_WRAPMARKER	(((uint32_t)SLT__Reserved << 24) | 0x575757) /* "WWW" */