Coalesce data and coding index

[carllin]

Problem

Data and Coding Index structures have the same signature and methods, can be merged into one.

Summary of Changes

Replace DataIndex and CodingIndex with single ShredIndex struct
Fixes #

[codecov]

Codecov Report

Merging #7765 into master will decrease coverage by <.1%.
The diff coverage is 40%.

@@           Coverage Diff            @@
##           master   #7765     +/-   ##
========================================
- Coverage    81.8%   81.8%   -0.1%     
========================================
  Files         241     241             
  Lines       50903   50882     -21     
========================================
- Hits        41652   41632     -20     
+ Misses       9251    9250      -1

Pull request has been modified.