v1.18: blockstore: relax parent slot meta check for clear_unconfirmed_slot

[AshwinSekar]

backport of the relevant parts in solana-labs#35124

[steviez]

I'm good with this minimized BP for v1.18. The previous code could panic from this previous expect:

.expect("Unconfirmed slot should have had parent slot set");

which is now an error message. While we shouldn't hit the panic in regular operation, it is technically recoverable since ReplayStage (the caller of this function) is going to repair the slot it just purged, which will in turn populate the parent slot meta that was unable to be found.

I'm going to add @willhickey as a reviewer; let's confirm that Will is onboard with the BP as well before pushing

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.6%. Comparing base (9b784a2) to head (a39e632).

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.18      #68     +/-   ##
=========================================
- Coverage    81.6%    81.6%   -0.1%     
=========================================
  Files         827      827             
  Lines      224306   224311      +5     
=========================================
- Hits       183128   183117     -11     
- Misses      41178    41194     +16     

[willhickey]

Approved for beta

[t-nelson]

in the future, try to make changes with potential backportability in mind. it is much safer to take backports that are unmodified when possible