proxy(tokio-postgres): refactor typeinfo query to occur earlier (#11993)

## Problem

For #11992 I realised we need to get the type info before executing the
query. This is important to know how to decode rows with custom types,
eg the following query:

```sql
CREATE TYPE foo AS ENUM ('foo','bar','baz');
SELECT ARRAY['foo'::foo, 'bar'::foo, 'baz'::foo] AS data;
```

Getting that to work was harder that it seems. The original
tokio-postgres setup has a split between `Client` and `Connection`,
where messages are passed between. Because multiple clients were
supported, each client message included a dedicated response channel.
Each request would be terminated by the `ReadyForQuery` message.

The flow I opted to use for parsing types early would not trigger a
`ReadyForQuery`. The flow is as follows:

```
PARSE ""    // parse the user provided query
DESCRIBE "" // describe the query, returning param/result type oids
FLUSH       // force postgres to flush the responses early

// wait for descriptions

  // check if we know the types, if we don't then
  // setup the typeinfo query and execute it against each OID:

  PARSE typeinfo    // prepare our typeinfo query
  DESCRIBE typeinfo
  FLUSH // force postgres to flush the responses early

  // wait for typeinfo statement

    // for each OID we don't know:
    BIND typeinfo
    EXECUTE
    FLUSH

    // wait for type info, might reveal more OIDs to inspect

  // close the typeinfo query, we cache the OID->type map and this is kinder to pgbouncer.
  CLOSE typeinfo 

// finally once we know all the OIDs:
BIND ""   // bind the user provided query - already parsed - to the user provided params
EXECUTE   // run the user provided query
SYNC      // commit the transaction
```

## Summary of changes

Please review commit by commit. The main challenge was allowing one
query to issue multiple sub-queries. To do this I first made sure that
the client could fully own the connection, which required removing any
shared client state. I then had to replace the way responses are sent to
the client, by using only a single permanent channel. This required some
additional effort to track which query is being processed. Lastly I had
to modify the query/typeinfo functions to not issue `sync` commands, so
it would fit into the desired flow above.

To note: the flow above does force an extra roundtrip into each query. I
don't know yet if this has a measurable latency overhead.

libs/proxy/tokio-postgres2/src/codec.rs

@@ -1,21 +1,16 @@
 use std::io;
 
-use bytes::{Buf, Bytes, BytesMut};
+use bytes::{Bytes, BytesMut};
 use fallible_iterator::FallibleIterator;
 use postgres_protocol2::message::backend;
-use postgres_protocol2::message::frontend::CopyData;
 use tokio_util::codec::{Decoder, Encoder};
 
 pub enum FrontendMessage {
     Raw(Bytes),
-    CopyData(CopyData<Box<dyn Buf + Send>>),
 }
 
 pub enum BackendMessage {
-    Normal {
-        messages: BackendMessages,
-        request_complete: bool,
-    },
+    Normal { messages: BackendMessages },
     Async(backend::Message),
 }
 
@@ -44,7 +39,6 @@ impl Encoder<FrontendMessage> for PostgresCodec {
     fn encode(&mut self, item: FrontendMessage, dst: &mut BytesMut) -> io::Result<()> {
         match item {
             FrontendMessage::Raw(buf) => dst.extend_from_slice(&buf),
-            FrontendMessage::CopyData(data) => data.write(dst),
         }
 
         Ok(())
@@ -57,7 +51,6 @@ impl Decoder for PostgresCodec {
 
     fn decode(&mut self, src: &mut BytesMut) -> Result<Option<BackendMessage>, io::Error> {
         let mut idx = 0;
-        let mut request_complete = false;
 
         while let Some(header) = backend::Header::parse(&src[idx..])? {
             let len = header.len() as usize + 1;
@@ -82,7 +75,6 @@ impl Decoder for PostgresCodec {
             idx += len;
 
             if header.tag() == backend::READY_FOR_QUERY_TAG {
-                request_complete = true;
                 break;
             }
         }
@@ -92,7 +84,6 @@ impl Decoder for PostgresCodec {
         } else {
             Ok(Some(BackendMessage::Normal {
                 messages: BackendMessages(src.split_to(idx)),
-                request_complete,
             }))
         }
     }