Add confirmed slots to EpochSlots

[carllin]

Problem

Dead slots are still marked as complete in ClusterSlots and repaired: #10082

Summary of Changes

Use later half of EpochSlots indexed 128..=255 to hold slots confirmed by the cluster. Confirmed slots are well suited for compression because they are usually monotonically increasing. This way unconfirmed dead slots with errors like InvalidTickCouont need not even attempt to be repaired.

1. Solves Invalid tick count causes slot dumping #10082
2. Contributes towards: Repairs can stall on a dead fork #10245
3. Aids in detecting future duplicate slots that have been confirmed and thus need to be replayed

Fixes #

[codecov]

Codecov Report

Merging #10246 into master will decrease coverage by 0.0%.
The diff coverage is 99.0%.

@@            Coverage Diff            @@
##           master   #10246     +/-   ##
=========================================
- Coverage    81.5%    81.4%   -0.1%     
=========================================
  Files         288      288             
  Lines       66608    66743    +135     
=========================================
+ Hits        54297    54350     +53     
- Misses      12311    12393     +82     

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