Event engine memory cache for nxt_sockaddr_t.

Introducing event engine memory cache and using the cache for
nxt_sockaddr_t structures.

src/nxt_conn.h

@@ -103,6 +103,7 @@ typedef struct {
 
     uint32_t                      ready;
     uint32_t                      batch;
+    uint32_t                      mem_cache;  /* 8 bits */
 
     /* An accept() interface is cached to minimize memory accesses. */
     nxt_work_handler_t            accept;