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other changes:
1) send proposal requests to the last proposal sender
2) add genericHeight to record the current block height in forest
3) send proposal requests when a proposal already exists but we need it's parent proposal
4) pull proposals from a random remote node witch blockHeight is higher than me

--bug=1003854 --bug=1003841

2. fix unit tests;

2. fix Engine.Verify check logic
3. remove redundancy check in handleProposal and fix logic to return
4. add check to jurge view has been committed

2. fix handleProposal to prune votes

2. Adjust code layout
3. add unit tests

2. add unit test;