Add a performance test for SyncProposalProcessor.

This patch adds a test to benchmark SyncProposalProcessor with various
implementations of the log interface and varying transaction size /
batch size. For now, there is only one implementation (SimpleLog).
Merge pull request #94 from m1ch1/log

Add a performance test for SyncProposalProcessor.
Flush the pipeline when a new follower joins in.
Merge pull request #93 from EasonLiao/flush_pipe

Fix the bug of synchronizing new joined follower in broadcasting phase