workload: refactor kv key generator

Refactor the KV workload's key generator interface to reduce the interface
surface area. The keyGenerator is now a struct, rather than an interface, and a
more constrained keyMapper interface encapsulates the logic necessary for
mapping the sequential index to the appropriate key. Key generator
configuration is held in a keyGeneratorConfig struct, while state that requires
mutual exclusion is held in a keyGeneratorState struct.

Epic: none
Release note: none

Author: jbowens