Fix Up Exits Pool Logic

[rauljordan]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

This is related to #7772. Our exits logic was needlessly complex, and we needed better tests for it. Additionally, we needed to simplify further parts of the code in there and ensure our binary search is correct according to the Go stdlib. Among the changes included are:

• Removal HasBeenIncluded() as the method was only used in tests
• No need for maps to keep track of exits. All we should do is keep an ordered list of validator indices for exits that are pending and check incoming exits against the list with binary search
• We should prevent malformed messages from making it into the pending exits list
• If we receive a new exit that has a more favorable exit epoch, we replace the existing pending exit

[codecov]

Codecov Report

Merging #7774 (d57bdb3) into master (660ed2d) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7774      +/-   ##
==========================================
+ Coverage   62.17%   62.24%   +0.07%     
==========================================
  Files         429      429              
  Lines       30366    30365       -1     
==========================================
+ Hits        18881    18902      +21     
+ Misses       8549     8527      -22     
  Partials     2936     2936