WIP: [ibft] Fix validator panic due to repeat notification in syncing

[DarianShawn]

Description

The validators will panic after syncing a bulk of blocks, due to incorrectly implementation of IBFT consensus context handling.
The consensus routine will be triggered multiple times even it is not thread-safe, and occasionally, the built block is clear before gossip it, then the server panic with nil object access.
The PR:

• Uses an atomic value to disable reentrant before last routine stopped.
  • To prevent reentrant, which will crash all states.
• Update validator set only when notified new block.
  • It is a better triggered point because it will never meet sequence problem before join in IBFT consensus cycle.

Changes include

• Hotfix (change that solves an urgent issue, and requires immediate attention)

Testing

• I have tested this code with the official test suite

[codecov-commenter]

Codecov Report

Merging #320 (ff0147a) into dev (d7c5a9e) will increase coverage by 0.28%.
The diff coverage is 28.46%.

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@@            Coverage Diff             @@
##              dev     #320      +/-   ##
==========================================
+ Coverage   47.74%   48.02%   +0.28%     
==========================================
  Files         132      132              
  Lines       20125    20103      -22     
==========================================
+ Hits         9608     9655      +47     
+ Misses       9684     9620      -64     
+ Partials      833      828       -5     

Impacted Files	Coverage Δ
consensus/ibft/currentstate/state.go	18.07% <0.00%> (+0.39%)	⬆️
consensus/ibft/snapshot.go	59.71% <0.00%> (-0.35%)	⬇️
protocol/client.go	48.61% <ø> (-0.61%)	⬇️
protocol/service.go	26.17% <ø> (+4.26%)	⬆️
consensus/ibft/ibft.go	23.41% <16.66%> (+0.33%)	⬆️
consensus/ibft/consensus.go	37.17% <44.00%> (+1.29%)	⬆️
protocol/syncer.go	49.49% <51.85%> (-0.12%)	⬇️

... and 4 files with indirect coverage changes

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[DarianShawn]

Tests pass, but manual tests fail when shutting down one of the 4 validator.
Blocks are no longer produced after stopping 1 validator, which is a disaster.
Stop this PR because we need to reinvent a real working solution for IBFT, instead of tinkering here and there.