psql-srv: Add integer_datetimes to PG ready message

A long, long time ago, in a galaxy far, far away, Postgres internally
represented `timestamp` and `interval` types as `doubles`. Around
version 8.4 (July 2009), this was changed to an `integer` for various
precision correctness issues.

As a result, Postgres would, and still does to this day, return a
`ParameterStatus` to clients on a new connection to declare if the
server is treating `timestamp` and `interval` types as `doubles`. This
status is named `integer_datetimes` and, since Postgres v10, always
returns `on` (meaning it's always returning 8-byte integers).

This distinction primarily matters then the server is asked to return
values in the binary format; text format just returns a string
representation of the value.

Some Postgres drivers still expect to see that `integer_datetimes`
parameter, and if they do not see it, will assume the server will
represent `timestamp` and `interval` types as `doubles`. This is what
happens with the PGJDBC driver. When the client asks for a `timestamp`
column, to be returned in binary format - which is what pgjdbc does
for named prepared statements - the connection will still check its
state to see if the `timestamp`'s 8-bytes should be interpreted as a
double or integer value (a `long`).

This CL adds an extra `ParameterStatus` in the new connection ready
message for the `integer_datetimes` parameter.

Fixes: #1365
Release-Note-Core: Fix bug affecting pgjdbc clients where
  binary-formatted timestamp columns in result sets are interpreted
  incorrectly.
Change-Id: Ia783b861b9dd8923e57a235cf097548c468648aa
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/7936
Tested-by: Buildkite CI
Reviewed-by: Johnathan Davis <jcd@readyset.io>

Author: jasobrown-rs

psql-srv/src/protocol.rs

@@ -223,6 +223,10 @@ impl Protocol {
                     parameter_name: "standard_conforming_strings".to_owned(),
                     parameter_value: "on".to_owned(),
                 },
+                BackendMessage::ParameterStatus {
+                    parameter_name: "integer_datetimes".to_owned(),
+                    parameter_value: "on".to_owned(),
+                },
                 BackendMessage::ParameterStatus {
                     parameter_name: "server_version".to_owned(),
                     parameter_value: version,
@@ -1382,6 +1386,7 @@ mod tests {
                     BackendMessage::ParameterStatus { .. },
                     BackendMessage::ParameterStatus { .. },
                     BackendMessage::ParameterStatus { .. },
+                    BackendMessage::ParameterStatus { .. },
                     BackendMessage::ReadyForQuery {
                         status: READY_FOR_QUERY_IDLE
                     }