[PATCH] Add support for specifying read/write mix workloads better

[PATCH] Add support for specifying read/write mix workloads better

Added options:

- rwmixcycle=x, use x msecs as the cycle period for read/write switches
- rwmixread=x, x% of ios will be reads
- rwmixwrite=x, x% of ios will be writes

Last given of the latter two will override the first.