Support both rate limiter and pendingPropagation in mempool

[goshawk-3]

fixes #1237

[codecov-commenter]

Codecov Report

Merging #1238 (ab6ad48) into master (828364b) will decrease coverage by 2.89%.
The diff coverage is 52.50%.

@@            Coverage Diff             @@
##           master    #1238      +/-   ##
==========================================
- Coverage   50.51%   47.62%   -2.90%     
==========================================
  Files         145      139       -6     
  Lines        7544     8508     +964     
==========================================
+ Hits         3811     4052     +241     
- Misses       3066     3870     +804     
+ Partials      667      586      -81     

Impacted Files	Coverage Δ
pkg/config/consts.go	0.00% <ø> (ø)
pkg/config/genesis/config.go	0.00% <0.00%> (ø)
pkg/core/candidate/validator.go	64.00% <ø> (+6.85%)	⬆️
pkg/core/chain/legacy.go	0.00% <0.00%> (-28.58%)	⬇️
pkg/core/chain/mock.go	9.09% <ø> (+0.75%)	⬆️
...kg/core/consensus/blockgenerator/candidate/mock.go	0.00% <0.00%> (ø)
pkg/core/consensus/comms.go	0.00% <0.00%> (ø)
pkg/core/consensus/fixtures.go	0.00% <0.00%> (ø)
pkg/core/consensus/header/header.go	24.50% <0.00%> (+2.41%)	⬆️
pkg/core/consensus/publisher.go	0.00% <0.00%> (ø)
... and 183 more

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[autholykos]

Some minor changes requested:

• If my understanding of the rate limiting is correct, we should probably allow an additional configuration parameter to tune transaction burst other than 1.
• Context cancelation leakage should be assessed. It might be that the m.pendingPropagation channel would be garbage collected, but this needs to be double checked

[autholykos]

LGTM