cosbench should support 100% read mode and mix mode with adjustable read and write ratios

Currently only mode supported is write
"read" mode will require a prepare stage to create objects

Also should support mix mode and allow user to specify read and write ratios

The issue names no files or tests; start by locating the cosbench mode-handling entry point and the prepare stage. Define the behavior for 100% read mode and mixed read/write ratios, then verify that users can configure both ratios and that read mode prepares objects before benchmarking.

Written by the indexing model from the issue text.