ACCUMULO-2947 Allow configuration of number of cells per mutation in mem stress test

A new switch to the memory stress test Write utility, --max-cells-per-mutation,
controls the maximum number of cells to be written in any mutation. If the maximum is
hit before the configured row width, the current mutation is used and the next mutation
picks up where it left off, in the same row.
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT