Allow BASE_BACKUP to be throttled

A new MAX_RATE option allows imposing a limit to the network transfer
rate from the server side.  This is useful to limit the stress that
taking a base backup has on the server.

pg_basebackup is now able to specify a value to the server, too.

Author: Antonin Houska

Patch reviewed by Stefan Radomski, Andres Freund, Zoltán Böszörményi,
Fujii Masao, and Álvaro Herrera.

src/backend/replication/repl_gram.y

@@ -69,6 +69,7 @@ Node *replication_parse_result;
 %token K_PROGRESS
 %token K_FAST
 %token K_NOWAIT
+%token K_MAX_RATE
 %token K_WAL
 %token K_TIMELINE
 %token K_PHYSICAL
@@ -113,7 +114,7 @@ identify_system:
 			;
 
 /*
- * BASE_BACKUP [LABEL '<label>'] [PROGRESS] [FAST] [WAL] [NOWAIT]
+ * BASE_BACKUP [LABEL '<label>'] [PROGRESS] [FAST] [WAL] [NOWAIT] [MAX_RATE %d]
  */
 base_backup:
 			K_BASE_BACKUP base_backup_opt_list
@@ -157,6 +158,11 @@ base_backup_opt:
 				  $$ = makeDefElem("nowait",
 								   (Node *)makeInteger(TRUE));
 				}
+			| K_MAX_RATE UCONST
+				{
+				  $$ = makeDefElem("max_rate",
+								   (Node *)makeInteger($2));
+				}
 			;
 
 /* CREATE_REPLICATION_SLOT SLOT slot PHYSICAL */