*: extend PreBlock processing callback

[AnnaShaleva]

The necessity of this extension is described in bane-labs/go-ethereum#301 (comment). We need at least M correct PreCommits to decrypt Envelopes content, but decryption may be checked only by using the group of PreCommits. Thus, we can't say whether a single PreCommit can/can't decrypt Envelopes content.

@roman-khimov, please, review it carefully, I checked what I could and there seems to be no tricks possible to skip verification, but maybe you'll fined something. May be it'll be easier to review if you take a look at bane-labs/go-ethereum@f4e830c.

[codecov]

Codecov Report

Attention: Patch coverage is 48.48485% with 17 lines in your changes missing coverage. Please review.

Project coverage is 59.25%. Comparing base (d574d48) to head (a6c5026).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
dbft.go	42.10%	8 Missing and 3 partials ⚠️
check.go	58.33%	4 Missing and 1 partial ⚠️
context.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   59.47%   59.25%   -0.22%     
==========================================
  Files          32       32              
  Lines        1821     1831      +10     
==========================================
+ Hits         1083     1085       +2     
- Misses        659      665       +6     
- Partials       79       81       +2     

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[AnnaShaleva]

I found one more bug.

[AnnaShaleva]

@roman-khimov, I placed encrypted data decoding to the wrong dBFT callback, so the library itself works correctly, the mistae was on the NeoX side. Ready to be merged.