pgx.Conn: Fix memory leak: Delete items from preparedStatements

Previously, items were never removed from the preparedStatements map.
This means workloads that send a large number of unique queries could
run out of memory. Delete items from the map when sending the
deallocate command to Postgres. Add a test to verify this works.

Fixes https://github.com/jackc/pgx/issues/1456

conn.go

@@ -1326,6 +1326,7 @@ func (c *Conn) deallocateInvalidatedCachedStatements(ctx context.Context) error
 
 	for _, sd := range invalidatedStatements {
 		pipeline.SendDeallocate(sd.Name)
+		delete(c.preparedStatements, sd.Name)
 	}
 
 	err := pipeline.Sync()