readyset-mysql: Disable statement cache

mysql_async has a "statement cache" feature, which limits the number of
prepared statements that exist on a given connection at one time. This
is enabled by default with a value of 32. When the number of prepared
statement exceeds 32, a previously-created prepared statement is
deallocated, which results in errors when a client attempts to execute
it.

This commit disables this statement cache to ensure that we can support
an arbitrary number of prepared statements (as we do for Postgres).

Release-Note-Core: Fixed an issue where the number of prepared
  statements for a MySQL connection was limited to 32
Change-Id: I1050e2df98a95a9d8c73c54b957ffd65e381b87b

readyset-mysql/src/upstream.rs

@@ -166,13 +166,17 @@ impl MySqlUpstream {
             .as_deref()
             .ok_or(ReadySetError::InvalidUpstreamDatabase)?;
 
-        let mut opts =
-            Opts::from_url(url).map_err(|e: UrlError| Error::MySql(mysql_async::Error::Url(e)))?;
+        let mut builder = {
+            let opts = Opts::from_url(url)
+                .map_err(|e: UrlError| Error::MySql(mysql_async::Error::Url(e)))?;
+            OptsBuilder::from_opts(opts).stmt_cache_size(0)
+        };
 
         if let Some(cert_path) = upstream_config.ssl_root_cert.clone() {
             let ssl_opts = SslOpts::default().with_root_certs(vec![cert_path.into()]);
-            opts = OptsBuilder::from_opts(opts).ssl_opts(ssl_opts).into();
+            builder = builder.ssl_opts(ssl_opts);
         }
+        let opts: Opts = builder.into();
 
         let span = info_span!(
             "Connecting to MySQL upstream",