Add --sync-safekeepers starting standalone walproposer to sync safekeepers (#439).

It is intended to solve the following problems:

a) Chicken-or-the-egg one: compute postgres needs data directory
   with non-rel files that are downloaded from pageserver by calling
   basebackup@LSN. This LSN is not arbitrary, it must include all
   previously committed transactions and defined through consensus
   voting, which happens... in walproposer, a part of compute node.

b) Just warranting such LSN is not enough, we must also actually commit
   it and make sure there is a safekeeper who knows this LSN is
   committed so WAL before it can be streamed to pageserver -- otherwise
   basebackup will hang waiting for WAL. Advancing commit_lsn without
   playing consensus game is impossible, so speculative 'let's just poll
   safekeepers, learn start LSN of future epoch and run basebackup'
   won't work.

Currently --sync-safekeepers is considered completed when 1) at least majority
of safekeepers and 2) *all* safekeepers with live connection to walproposer
switch to new epoch and advance commit_lsn allowing basebackup to proceed. 2)
limits availablity, but that's because currently we don't have a mechanism
defining which safekeeper should stream WAL into pageserver.

Author: knizhnik

src/backend/main/main.c

@@ -35,6 +35,7 @@
 #include "common/username.h"
 #include "port/atomics.h"
 #include "postmaster/postmaster.h"
+#include "replication/walproposer.h"
 #include "storage/spin.h"
 #include "tcop/tcopprot.h"
 #include "utils/help_config.h"
@@ -209,6 +210,8 @@ main(int argc, char *argv[])
 		WalRedoMain(argc, argv,
 					 NULL,		/* no dbname */
 					 strdup(get_user_name_or_exit(progname)));	/* does not return */
+	else if (argc > 1 && strcmp(argv[1], "--sync-safekeepers") == 0)
+		WalProposerSync(argc, argv);
 	else
 		PostmasterMain(argc, argv); /* does not return */
 	abort();					/* should not get here */