Refactor/writecompletion

This set of commits enables a post processing path for the retrieveN command. It's required to allow the rocksdb back-end to cleanup allocated resources. For the in-memory back-ends it's a no-op.

Main new things:
* extended the internal back-end API by a post-processing call
* added new command-specific handling of rdma-write completion events
* adjusted the retrieveN state machine to handle write completion events