Don't warn when a connection closes without sending data

[markmnl]

Problem

TCP health checks, load balancer probes and port scanners connect to the fmsg port, sometimes complete the TLS handshake, and then close without sending anything. Each of these logged:

WARN: reading header from, 203.0.113.7:51234: EOF

With a health check running every minute from several regions, these warnings flood the log and bury real ones.

Change

• responseTrackingConn now also counts the bytes read from the connection.
• When readHeader fails before any bytes arrived, and the error is io.EOF or a connection reset, handleConn logs it at INFO (INFO: <addr> closed the connection without sending data) instead of as a warning, so operators still see who connected and from where.
• Everything else still warns exactly as before:
  • a partial or malformed header;
  • a read timeout;
  • a non-TLS client;
  • any other TLS or read error.

Tests

cmd/fmsgd/empty_conn_test.go, run against a real TLS listener:

• A plain TCP connect-and-close doesn't warn.
• A completed TLS handshake followed by a close doesn't warn.
• A connection that sends only the version byte and then closes still warns.
• A table test for closedWithoutData: EOF or reset before any data counts as a silent close. EOF after some data, a timeout, or other errors don't.

With the check disabled, the two probe tests fail, so the tests do catch the old behaviour.

go vet ./... and go test ./... pass.

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