Run batch requests concurrently

[sujeetsr]

Enable batch requests in dynamic schema to run concurrently. This is just adding the same fix in 411239f4b35e1e72e8cedb6fcc834cc05b4e301a to executor so that it applies to dynamic schema too.

@sunli829 , is the execute_batch function in src/schema.rs not needed anymore with this change, because that implements Executor too?

[sunli829]

Thanks