Change detection and updating in Slasher to per attestation

[0xKiwi]

Part of #4836

This PR modifies how updating and detection is performed on the slasher by changing it to perform the actions on a per attestation basis, rather than a per validating indice basis.

This gives us less overhead, less DB locks, and makes detection much simpler.

[codecov]

Codecov Report

Merging #5043 into master will decrease coverage by 33.85%.
The diff coverage is 77.90%.

@@             Coverage Diff             @@
##           master    #5043       +/-   ##
===========================================
- Coverage   44.08%   10.23%   -33.86%     
===========================================
  Files         214       99      -115     
  Lines       16683     7444     -9239     
===========================================
- Hits         7355      762     -6593     
+ Misses       8031     6586     -1445     
+ Partials     1297       96     -1201