Make walsender options order-independent

While doing this, also move base backup options into
a struct instead of increasing the number of parameters
to multiple functions for each new option.

src/include/replication/basebackup.h

@@ -12,6 +12,8 @@
 #ifndef _BASEBACKUP_H
 #define _BASEBACKUP_H
 
-extern void SendBaseBackup(const char *backup_label, bool progress, bool fastcheckpoint);
+#include "replication/replnodes.h"
+
+extern void SendBaseBackup(BaseBackupCmd *cmd);
 
 #endif   /* _BASEBACKUP_H */

src/include/replication/replnodes.h

@@ -45,9 +45,7 @@ typedef struct IdentifySystemCmd
 typedef struct BaseBackupCmd
 {
 	NodeTag		type;
-	char	   *label;
-	bool		progress;
-	bool		fastcheckpoint;
+	List	   *options;
 }	BaseBackupCmd;