QCLIENT: Allow persistency layer to be configurable

This changeset depends on qclient changes eos/qclient!7 which allows for the persistency layer to be configurable. Also the flusher interface is made in a way where one can configure the serial behaviour which is used currently or a lockfree behaviour where in the persistency layer is delegated the responsibility of bookkeeping requests and responses. There are tests to demonstrate various behaviours and it is possible to switch at restart what persistency layer is needed. Currently we only mark ROCKSDB & ROCKSDB_MULTI as persistency layers that will be supported though there is a memory implementation in place which can be journalled and easily modified in the future.