Feature: implement NegotiateProtocolVersion message

NegotiateProtocolVersion message is a message sent from backend to
frontend. After a startup message is sent from frontend to backend,
and the minor protocol version in the startup message is not supported
by the backend, the backend sends back a NegotiateProtocolVersion
message along with the latest minor protocol version it supports.

For example, PostgreSQL 18 client could request a protocol version 3.2
against PostgreSQL 17 server, then the PG17 server sends back the
NegotiateProtocolVersion message with protocol version 3.0 since it
does not support 3.2. If the PG18 client accepts it, then from now on
the protocol version 3.0 is used for the communication.

At this point, as PG18's psql (or libpq) uses protocol version 3.0 by
default, PG18 (or before) server will not send back the
NegotiateProtocolVersion message because the versions support 3.0.

The only case when 3.2 is used, is PG18's psql (or libpq) uses a
connection option max_protocol_version (or environment variable
PGMAXPROTOCOLVERSION) being set to "3.2". I don't know if the case
happen in the wild after PG18 is released, but I would like to prepare
for the situation by implementing NegotiateProtocolVersion message in
Pgpool-II 4.7. Attached patch does followings for the purpose.

- Add ProcessNegotiateProtocol() to pool_do_auth() to process the
message while handling a startup message. It reads the message from
all backend, then forward it to frontend. The message is saved in
ConnectionInfo for the case below. It also save the major and minor
protocol version after negotiation to POOL_CONNECTION_POOL_SLOT for
later use. The version numbers are also saved to ConnectionInfo so
that pcp_proc_info shows them.

- When a client connects to pgpool, it looks for cached connections in
the connection: pgpool tries to match the startup message with the
one in the connection pool. Since the startup message in the
connection pool is saved at the time when the client connects to
pgpool, it is possible that two connection cache are created with
same user and database. Suppose there's a connection cache with a
startup message having protocol version 3.0, and a new client tries
to connect to pgpool using protocol version 3.2. Pgpool looks for a
connection cache with the startup message having protocol version
3.2, not 3.0. As a result a new connection cache entry is created
with protocol version 3.2.

- When a client uses the connection pool, the saved
NegotiateProtocolVersion message is sent to frontend to emulate the
protocol negotiation.

- The frontend/backend protocol 3.2 changes the BackendKeyData message
format but it's not implemented in this patch yet. I will work on it
later on.

Best regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp