* types: change `Commit` to consist of just signatures
These are final changes towards removing votes from commit and leaving
only signatures (see ADR-25)
Fixes#1648
* bring back TestCommitToVoteSetWithVotesForAnotherBlockOrNilBlock
+ add absent flag to Vote to indicate that it's for another block
* encode nil votes as CommitSig with BlockIDFlagAbsent
+ make Commit#Precommits array of non-pointers
because precommit will never be nil
* add NewCommitSigAbsent and Absent() funcs
* uncomment validation in CommitSig#ValidateBasic
* add comments to ValidatorSet funcs
* add a changelog entry
* break instead of continue
continue does not make sense in these cases
* types: rename Commit#Precommits to Signatures
* swagger: fix /commit response
* swagger: change block_id_flag type
* fix merge conflicts