Implement NOWAIT option for BASE_BACKUP command

Specifying this option makes the server not wait for the
xlog to be archived, or emit a warning that it can't,
instead leaving the responsibility with the client.

This is useful when the log is being streamed using
the streaming protocol in parallel with the backup,
without having log archiving enabled.

src/backend/replication/repl_gram.y

@@ -71,6 +71,7 @@ Node *replication_parse_result;
 %token K_LABEL
 %token K_PROGRESS
 %token K_FAST
+%token K_NOWAIT
 %token K_WAL
 %token K_START_REPLICATION
 
@@ -107,7 +108,7 @@ identify_system:
 			;
 
 /*
- * BASE_BACKUP [LABEL '<label>'] [PROGRESS] [FAST] [WAL]
+ * BASE_BACKUP [LABEL '<label>'] [PROGRESS] [FAST] [WAL] [NOWAIT]
  */
 base_backup:
 			K_BASE_BACKUP base_backup_opt_list
@@ -142,6 +143,11 @@ base_backup_opt:
 				  $$ = makeDefElem("wal",
 						   (Node *)makeInteger(TRUE));
 				}
+			| K_NOWAIT
+				{
+				  $$ = makeDefElem("nowait",
+						   (Node *)makeInteger(TRUE));
+				}
 			;
 
 /*