v1.18: RPC sendTransaction: be more liberal in determining and setting last_valid_block_height for skip_preflight case (backport of #483)

[mergify]

Problem

Transactions submitted to RPC sendTransaction with skipPreflight: true frequently aren't retried by the SendTransactionService (STS). This happens when the transaction's recent blockhash is newer than the RPC node's finalized bank, because unless preflightCommitment is speficied, the RPC handler uses the default finalized bank to find a last_valid_block_height to provide to the STS. If the blockhash cannot be found (as when too new), last_valid_block_height is set to 0, and the STS only broadcasts the transaction once.

Summary of Changes

• If skip_preflight is set, use the processed commitment Bank to search for the recent blockhash.
• If the last_valid_block_height still cannot be determined, use the durable-nonce logic to retry the transaction for an arbitrary, fixed number of blocks (skip_preflight being an indication that a client wants their transactions to be sprayed around regardless of likelihood of success).

Fixes #479

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This is an automatic backport of pull request #483 done by [Mergify](https://mergify.com).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.5%. Comparing base (2444794) to head (48f347d).

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.18     #873     +/-   ##
=========================================
- Coverage    81.5%    81.5%   -0.1%     
=========================================
  Files         827      827             
  Lines      224845   224848      +3     
=========================================
- Hits       183470   183446     -24     
- Misses      41375    41402     +27     

[t-nelson]

oh yeah... forgot about this one.