Support splitting tablet with lots of files

[keith-turner]

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

Currently in the main branch for Accumulo 4.0 there is no support for splitting tablets with lots of files. This exist in earlier versions of Accumulo but was not carried forward. Need to readd support for this.

Describe the solution you'd like

In earlier version of accumulo when a tablet had lots of files the split code would merge the rfile indexes in batches and then iterated over this merged view. For example if a tablet had 1000 files it might read 200 files at time merging their indexes into a new file, then it would read over the intermediate merged index files to compute the split point.