Add additional ensemble tests to test peer syncing

Add ensemble_basic4, ensemble_sync, and ensemble_interleave tests.

ensemble_sync tests the new AAE-based peer syncing logic. The test
checks various scenarios with different levels of data corruption.

ensemble_interleave tests a specific scenario where two peers become
corrupted one after the other. This tests the scenario where the
second peer becomes untrusted while the first peer may be syncing
with it.
Merge pull request #573 from basho/jdb-ensemble-sync