Support async Get / GetMeta / Add / Delete APIs

Protocol extensions will be required to support heavy DGM in a more scalable way and use connection sockets much more efficiently.

As a starting point and short-term solution, we need to provide async types of operations for Get / GetMeta / Add / Delete APIs. Note that those extensions will mitigate the performance regression from the full ejection cache management.

More details can be found on

https://docs.google.com/document/d/1tJYCW_sPbjqQD-X7LLAb88LNZwYmODGn-awxYrPHQeg/edit#

As a long-term solution, we need to revisit the overall protocol specs and extend them to address the above issues completely.