Create utility to presplit the fate table

[keith-turner]

Is your feature request related to a problem? Please describe.

In order to spread the new fate table over multiple tablet servers the ability to pre-split it is needed. This functionality would be useful if multiple processes executed fate operations as proposed in #4978.

Describe the solution you'd like

Create a fate admin command that can generate N split for the fate table where those splits work well with the fate schema to evenly distribute fate operations between the splits. In the command help document how it can used with shell commands to split that table and why it would be used.

[keith-turner]

Could also include the scanref table in this work. Both may use a uuid for the row, so can generate good splits for both using the same code.