Prepared Statement connection is active and wait for ClientRead.

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Hi team,

Right now, asyncpg does prepared statements in two steps.
1. Parse, Describe, Flush.
2. Bind, Execute, Sync.

Two coroutines represent these two steps. In a highly concurrent setup, the wall-clock gap between these two steps can be large. Why is it causing a problem for me? Command `Flush` does not change the connection state, i.e., `active`. So I observe a lot of queries show up as
```
wait_event_type | wait_event | state
-----------------+------------+--------
Client | ClientRead | active
```
in `pg_stat_activity`. This causes a lot of unnecessary confusion for our database monitors/observability tools. The connection is essentially idle between these two steps, so I think it is better to report it as `wait_event=ClientRead` but `state=idle`? Meanwhile, the command `Sync` will change the connection state to `idle`, so after step 2, the connection becomes idle.

My ask is: could we use `Sync` instead of `Flush` in step 1?

Moreover, there could be more than one `bind` step, so the sequence could be
```
1. Parse, Describe, Flush.
2. Bind, Execute, Sync.
3. Bind, Execute, Sync.
...
```
In this case, the connection is active only between step 1 and step 2, but idle for all other step gaps. I kind of feel this behavior is inconsistent.
I might have neglected some basic design about PostgreSQL [extended query](https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY). Hope to hear from you soon!

Beginne mit dem im Issue beschriebenen Ablauf für vorbereitete Statements von asyncpg und der Dokumentation zum Extended-Query-Protokoll von PostgreSQL. Ermittle, ob eine Änderung der Flush/Sync-Sequenz die Korrektheit bei mehreren Bind/Execute-Schritten bewahren kann; abgeschlossen ist die Aufgabe, wenn pg_stat_activity den Verbindungsstatus konsistent meldet, ohne vorbereitete Statements zu beeinträchtigen.

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