Add support for a composite read/write transaction to fixed asset

[davidkel]

As part of performance benchmarking hyperledger fabric there is a need for a composite transaction that addresses the following scenario

1. create a set number of keys (fixed asset size)
2. read x number of existing keys
3. write y number to existing keys
4. option to choose whether the write is done to keys which have been read, explicitly not read, completely random

The ability to create a set number of initial keys for use by the subsequent transaction should be done via a round definition rather than through the workload module initialisation