Governable minimum gas price limit

[uprendis]

• SFC contract is authorized to change the minimum gas price limit. Previously minimum gas price limit was set to 1 gwei
• SFC contract is authorized to shut down non-upgraded nodes below a specified version

SFC contract is expected to be owned by the governance contract, hence the actions above are decentralized

[codecov-commenter]

Codecov Report

Merging #539 into develop2 will increase coverage by 2.82%. The diff coverage is 2.27%.

@@             Coverage Diff              @@
##           develop2     #539      +/-   ##
============================================
+ Coverage     40.98%   43.80%   +2.82%     
============================================
  Files           160      124      -36     
  Lines          9799     6711    -3088     
============================================
- Hits           4016     2940    -1076     
+ Misses         5410     3524    -1886     
+ Partials        373      247     -126     

Impacted Files	Coverage Δ
app/store_migration.go	0.00% <0.00%> (ø)
app/store_sfc_constants.go	0.00% <0.00%> (ø)
cmd/lachesis/config.go	6.40% <ø> (+0.10%)	:arrow_up:
cmd/lachesis/main.go	51.98% <ø> (-0.24%)	:arrow_down:
cmd/tx-storm/acc.go	23.80% <ø> (ø)
evmcore/tx_pool.go	73.88% <12.50%> (-1.32%)	:arrow_down:
inter/ascii_scheme.go	80.00% <0.00%> (-2.54%)	:arrow_down:
utils/uint2hash.go	0.00% <0.00%> (ø)
gossip/serverpool.go
... and 26 more

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[uprendis]

Already included in #540