MDEV-15914: Fast path for rw_trx_hash::find()

rw_trx_hash_t::find(): Implement a fast path for looking up
the current transaction. This helps transactions that visit
rows modified by themselves.

storage/innobase/include/trx0sys.h

@@ -627,6 +627,12 @@ class rw_trx_hash_t
     */
     if (!trx_id)
       return NULL;
+    if (caller_trx && caller_trx->id == trx_id)
+    {
+      if (do_ref_count)
+        caller_trx->reference();
+      return caller_trx;
+    }
 
     trx_t *trx= 0;
     LF_PINS *pins= caller_trx ? get_pins(caller_trx) : lf_hash_get_pins(&hash);