fix: drop SAP statement after prepare per Hana client docs

each statement takes up a pool so if you query frequently you can
exhaust that pool of statement handles.  this drops the statement
after we're done with it so we don't exhaust the pool of statements

src/driver/sap/SapQueryRunner.ts

@@ -181,6 +181,18 @@ export class SapQueryRunner extends BaseQueryRunner implements QueryRunner {
             await Promise.all(otherWaitingPromises);
         }
 
+        let statement: any;
+
+        const dropStatement = async () => {
+            await new Promise<void>((ok, fail) => {
+                if (!statement) {
+                    ok();
+                }
+
+                statement.drop(() => ok());
+            })
+        }
+
         const promise = new Promise(async (ok, fail) => {
            try {
                const databaseConnection = await this.connect();
@@ -190,7 +202,7 @@ export class SapQueryRunner extends BaseQueryRunner implements QueryRunner {
                const queryStartTime = +new Date();
                const isInsertQuery = query.substr(0, 11) === "INSERT INTO";
 
-               const statement = databaseConnection.prepare(query);
+               statement = databaseConnection.prepare(query);
                statement.exec(parameters, (err: any, result: any) => {
 
                    // log slow queries if maxQueryExecution time is set
@@ -237,8 +249,8 @@ export class SapQueryRunner extends BaseQueryRunner implements QueryRunner {
                });
            } catch (err) {
                 fail(err);
-            }
-        });
+           }
+        }).then(dropStatement, dropStatement)
 
         // with this condition, Promise.all causes unexpected behavior.
         // if (this.isTransactionActive)