Add MDB_NORDLOCK to omit all reader table usage

Calling app wants to manage its own locking.

libraries/liblmdb/lmdb.h

@@ -269,6 +269,8 @@ typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *rel
 #define MDB_MAPASYNC		0x100000
 	/** tie reader locktable slots to #MDB_txn objects instead of to threads */
 #define MDB_NOTLS		0x200000
+	/** don't use reader locktable at all, caller must manage read/write concurrency */
+#define MDB_NORDLOCK		0x400000
 /** @} */
 
 /**	@defgroup	mdb_dbi_open	Database Flags

libraries/liblmdb/mdb.c

@@ -3983,7 +3983,7 @@ mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode
 		goto leave;
 	}
 
-	if (F_ISSET(flags, MDB_RDONLY)) {
+	if ((flags & (MDB_RDONLY|MDB_NORDLOCK)) == MDB_RDONLY) {
 		rc = mdb_env_setup_locks(env, lpath, mode, &excl);
 		if (rc)
 			goto leave;