poc: new TxHandler decoder

[algorandskiy]

Summary

Use msgp marshaller instead of reflect in TxHandler

BenchmarkTxHandlerDecoder-8       	      68	  17116004 ns/op	 3335105 B/op	   51105 allocs/op
BenchmarkTxHandlerDecoderMsgp-8   	     122	  10213123 ns/op	 1189860 B/op	     614 allocs/op

BenchmarkTxHandlerProcessDecoded-8   	   62826	     19184 ns/op	    8956 B/op	      57 allocs/op
vs old
BenchmarkTxHandlerProcessDecoded-8   	   47414	     21560 ns/op	    8957 B/op	      57 allocs/op

Test Plan

TODO: write tests

[codecov[bot]]

Codecov Report

Merging #4266 (1a12135) into master (bf7aaec) will increase coverage by 0.03%. The diff coverage is 42.30%.

@@            Coverage Diff             @@
##           master    #4266      +/-   ##
==========================================
+ Coverage   55.18%   55.22%   +0.03%     
==========================================
  Files         398      398              
  Lines       50137    50148      +11     
==========================================
+ Hits        27669    27692      +23     
+ Misses      20156    20141      -15     
- Partials     2312     2315       +3     

Impacted Files	Coverage Δ
cmd/goal/clerk.go	8.75% <0.00%> (ø)
cmd/goal/multisig.go	10.00% <0.00%> (ø)
protocol/codec_tester.go	10.61% <0.00%> (ø)
data/txHandler.go	11.11% <20.00%> (+11.11%)	:arrow_up:
protocol/codec.go	64.21% <80.00%> (+1.85%)	:arrow_up:
cmd/tealdbg/local.go	73.95% <100.00%> (ø)
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	:arrow_down:
ledger/blockqueue.go	84.48% <0.00%> (-1.15%)	:arrow_down:
data/transactions/verify/txn.go	43.85% <0.00%> (-0.88%)	:arrow_down:
ledger/acctupdates.go	69.29% <0.00%> (-0.60%)	:arrow_down:
... and 8 more

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[algorandskiy]

Thank you for the feedback, I'll add a protocol/codec random data/length unit test and merge.

[algorandskiy]

Rebased/resolved a merge conflict