core, txpool: less allocations when handling transactions(revert)

[ucwong]

No description provided.

[codecov]

Codecov Report

Merging #672 into master will decrease coverage by 0.02%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #672      +/-   ##
==========================================
- Coverage   50.87%   50.84%   -0.03%     
==========================================
  Files         408      408              
  Lines       51907    51908       +1     
==========================================
- Hits        26409    26394      -15     
- Misses      23466    23477      +11     
- Partials     2032     2037       +5     

Impacted Files	Coverage Δ
core/types/transaction.go	55.73% <0.00%> (ø)
common/math/integer.go	93.33% <100.00%> (+0.22%)	⬆️
p2p/simulations/mocker.go	33.63% <0.00%> (-4.55%)	⬇️
ctxc/downloader/statesync.go	61.81% <0.00%> (-3.15%)	⬇️
whisper/whisperv6/message.go	69.94% <0.00%> (-1.16%)	⬇️
ctxc/downloader/downloader.go	78.32% <0.00%> (-0.66%)	⬇️
p2p/rlpx.go	71.38% <0.00%> (-0.57%)	⬇️
p2p/simulations/http.go	69.56% <0.00%> (-0.55%)	⬇️
p2p/discover/v4_udp.go	73.28% <0.00%> (-0.51%)	⬇️
p2p/server.go	66.98% <0.00%> (-0.39%)	⬇️
... and 3 more