yugabyte#4813 [YSQL] Suppress 'incomplete startup packet' log message

Summary:
Postgres logs each failed connection attempt

```
2019-11-28 13:24:26.501 UTC [15168] LOG: incomplete startup packet
```

Different monitoring tools may scans opened ports without sending any data.
Postgres log file will be flooded with above message in this case.
Solution is to avoid logging in case connection has no data.

Additional details can be found here: https://www.percona.com/blog/2019/12/03/postgresql-12-improvement-benign-log-entries-incomplete-startup-packet/
Vanilla postgres patch: https://git.postgresql.org/gitweb/?p=postgresql.git;a=patch;h=342cb650e

Test Plan:
Manual:
- start YB cluster with ysql enabled
- run simple port monitoring `for i in {1..100}; do nc -zv localhost 5433 ; done`
- check absence of `incomplete startup packet` log `cd ~/yugabyte-data && grep -e "incomplete startup packet" -R`

Reviewers: alex, neha

Reviewed By: neha

Subscribers: mihnea, mikhail, yql

Differential Revision: https://phabricator.dev.yugabyte.com/D8722

Author: d-uspenskiy

src/postgres/src/backend/postmaster/postmaster.c

@@ -1940,6 +1940,23 @@ ProcessStartupPacket(Port *port, bool SSLdone)
 	MemoryContext oldcontext;
 
 	pq_startmsgread();
+
+	/* Check we have no data at all. */
+	if (pq_peekbyte() == EOF)
+	{
+		/*
+		 * If we get no data at all, don't clutter the log with a complaint;
+		 * such cases often occur for legitimate reasons.  An example is that
+		 * we might be here after responding to NEGOTIATE_SSL_CODE, and if the
+		 * client didn't like our response, it'll probably just drop the
+		 * connection.  Service-monitoring software also often just opens and
+		 * closes a connection without sending anything.  (So do port
+		 * scanners, which may be less benign, but it's not really our job to
+		 * notice those.)
+		 */
+		return STATUS_ERROR;
+	}
+
 	if (pq_getbytes((char *) &len, 4) == EOF)
 	{
 		/*