v1.18: blockstore: store first received coding shred index in ErasureMeta (backport of #961)

[mergify]

Reuse the __unused_size : usize field in ErasureMeta to store the index of the first received coding shred for the FEC set.

Split from #835
Contributes to solana-labs#34897

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This is an automatic backport of pull request #961 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f133697 has failed:

On branch mergify/bp/v1.18/pr-961
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Unmerged paths:
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	both modified:   ledger/src/blockstore_meta.rs

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To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.5%. Comparing base (5485ec1) to head (5485ec1).

❗ Current head 5485ec1 differs from pull request most recent head 1d69535. Consider uploading reports for the commit 1d69535 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            v1.18     #981   +/-   ##
=======================================
  Coverage    81.5%    81.5%           
=======================================
  Files         827      827           
  Lines      224853   224853           
=======================================
  Hits       183467   183467           
  Misses      41386    41386