fix: fixing splitting logic

[mutianf]

The current split logic doesn't work well on very large tables because it caps the max number of splits too low and will merge tablets instead of using desired bundle size.

The updated logic will do the following:

1. Get keys from sample row keys
2. If the tablet is < desired bundle size, merge the tablets
3. If step 2 generated more than 5 million splits, merge more tablets to cap the abosulte max number of splits to 5M.