Flush pending writes on connection close

[mbutrovich]

In theory this should just be error messages and not partial query data, but I don't completely understand the network state machine. This is mostly to make sure we flush error messages on connection startup. See PostgresProtocolInterpreter::ProcessStartup's error states.

This came from @lmwnshn seeing behavior where clients couldn't connect while a simultaneous transaction was running that made DDL changes. We now get the verbose error (that was always written to the packet, just never flushed) returned to the client.

Client 1:

$ psql -h localhost -p 15721 -d noisepage
psql (12.7 (Ubuntu 12.7-0ubuntu0.20.04.1), server 9.5devel)
Type "help" for help.

noisepage=# begin;
BEGIN
Time: 1.427 ms
noisepage=# create table foo (id int);
CREATE TABLE
Time: 7.825 ms
noisepage=# 

Client 2:

$ psql -h localhost -p 15721 -d noisepage
psql: error: FATAL:  Failed to create a temporary namespace for this connection. There may be a concurrent DDL change. Please retry.