Blockstore: clean/save old TransactionMemos sensibly

[CriesofCarrots]

Problem

As of this commit, the compaction-filter purge includes the TransactionMemos column. For new TransactionMemos that include Slot in the key, the filter works as expected, and purges slots older than oldest_slot. However, old TransactionMemos that don't include Slot in the key are currently set to all be purged on the first periodic cleanup. This is because C::slot(C::index()) always returns 0 for these entries.

Summary of Changes

Add a field to OldestSlot to track when slot-0 entries should be cleaned
Plumb into PurgedSlotFilter

[codecov]

Codecov Report

Merging #33678 (d62ca06) into master (ac788ab) will increase coverage by 0.0%.
Report is 1 commits behind head on master.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #33678    +/-   ##
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  Coverage    81.8%    81.8%            
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  Files         806      806            
  Lines      217588   217673    +85     
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+ Hits       178106   178206   +100     
+ Misses      39482    39467    -15