This change implements the logic for the PrepareProposal ABCI++ method call. The main logic for creating and issuing the PrepareProposal request lives in execution.go and is tested in a set of new tests in execution_test.go. This change also updates the mempool mock to use a mockery generated version and removes much of the plumbing for the no longer used ABCIResponses.
Our test cases spew a lot of files and directories around $TMPDIR. Make more
thorough use of the testing package's TempDir methods to ensure these are
cleaned up.
In a few cases, this required plumbing test contexts through existing helper
code. In a couple places an explicit path was required, to work around cases
where we do global setup during a TestMain function. Those cases probably
deserve more thorough cleansing (preferably with fire), but for now I have just
worked around it to keep focused on the cleanup.
