Wrapping libunit's malloc() and free() calls for logging purposes.

This change aids heap usage analysis in applications.
The alloc and free functions are also required for lvlhash due to the upcoming
threading support, because using main nxt_memalign() and nxt_free() isn't safe
in a multithreaded app environment.  The reason is that these functions may use
thread-local structures which aren't initialized properly in applications.

src/nxt_lvlhsh.c

@@ -1015,17 +1015,3 @@ nxt_lvlhsh_retrieve(nxt_lvlhsh_t *lh, const nxt_lvlhsh_proto_t *proto,
 
     return NULL;
 }
-
-
-void *
-nxt_lvlhsh_alloc(void *data, size_t size)
-{
-    return nxt_memalign(size, size);
-}
-
-
-void
-nxt_lvlhsh_free(void *data, void *p)
-{
-    nxt_free(p);
-}