api: optimize /transactions/pending/{txid} endpoint

[algorandskiy]

Summary

• Use txtail data to find txid in memory (only txns with LastValid > current round) in addition to iterating blocks
• RPS: before: 10, after: 450
• Another optimization possible with caching blocks that would bump RPS to 30k
• It is possible to completely get rid of blocks iteration by one of the following:
  • Extend txtail to have MaxTxnLife old rounds. Requires lots of memory.
  • Make a new cache in AlgorandFullNode for txids: 26k txns/block * 1000 rounds => need 25 bits for all combos => 4 bytes (32bits) of txid instead of 32 bytes of a full txid would require 26M*4b = 100MB of RAM.
• Estimated extra memory footprint is 52MB = 26k txns/block * 1000 rounds * 2 bytes per round delta.

Test Plan

Added unit test for new txtail logic

[jannotti]

I like how simple this change is!

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (3a80a40) 55.92% compared to head (002cebf) 55.89%.
Report is 15 commits behind head on master.

Files	Patch %	Lines
node/node.go	0.00%	11 Missing ⚠️
ledger/txtail.go	88.46%	2 Missing and 1 partial ⚠️
ledger/ledger.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5891      +/-   ##
==========================================
- Coverage   55.92%   55.89%   -0.04%     
==========================================
  Files         477      477              
  Lines       67432    67458      +26     
==========================================
- Hits        37713    37703      -10     
- Misses      27164    27195      +31     
- Partials     2555     2560       +5     

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[algorandskiy]

While unit testing I found t.lastValid is aggressively trimmed and does not have all txids for MaxTxnLife rounds back, it only has txids for txns with LastValid in future from current.

[algorandskiy]

After offline discussion decided to still have it and use txTail lookup as a filter that probably vast majority of use cases when people immediately lookup for recently submitted transactions with LastValid at least few rounds in future.

[algorandskiy]

Relevant ticket #3967

[gmalouf]

Added a question or two, but overall this makes sense to me. I second JJ's suggestion around uint vs int, but will not block approval on it.