Add support for control of the read/write ratio applied to "mixed" mode testing

[jdonenine]

When running with the query operation mode (-m --mode) set to "mixed" (the default) the operations will be balanced evenly between the two - we'd like the ability to control the ratio of reads-to-writes that are applied during the testing.

The mode setting currently provides a high-level idea of this with the "read"-only and "write"-only modes.

This could be made as an optional parameter that is applied when the mode is "mixed" and its default could be left that 50 to represent the current state and remain backwards compatible.

[dkropachev]

Dup of #335