sql: statement diagnostics never finish for (SQL-level) prepared statements

[michae2]

While investigating a performance problem, @yuzefovich discovered that statement diagnostics requests are never completed for statements prepared and executed with the SQL PREPARE and EXECUTE commands. (Note that these SQL commands are different from the prepare and execute messages in the protocol used by most ORMs and applications.)

To reproduce:

1. In a SQL shell, execute a simple prepared statement:
   CREATE TABLE foo (f INT PRIMARY KEY);
PREPARE sel AS SELECT * FROM foo WHERE f = $1;
EXECUTE sel(0);
2. In the UI, find the SELECT * FROM foo WHERE f = $1 statement in the UI under "Statements".
3. Click Diagnostics > Activate Diagnostics
4. In a new SQL session, prepare and execute it again:
   PREPARE sel AS SELECT * FROM foo WHERE f = $1;
EXECUTE sel(0);
5. In the UI, the diagnostics status will still be WAITING.

This was on 21.1.1.