verify that handoff works in a mixed-version cluster

A problem with mixed-version handoff is described in basho/riak#407.
The verify_handof_mixed test in this commit demonstrates the issue(s).
always upgrade to 'current' from 'upgrade_version'

The basho builders have defined versions 'current', 'previous', and 'legacy'.
Instead of the odd configuration in the previous commit, upgrade from
'previous' to 'current' by default, and allow the test runner to use the
'upgrade_version' metadata to specify 'legacy' (or anything else) instead
of 'previous'. (thanks @jaredmorrow)
learn old fold version from old node before joining

Just a bit of future-proofing for when 'previous' or 'legacy' uses the 'v2' req.
Grab the fold req version from the old node before joining, so we know which
version we're waiting for the renegotiation to settle on.
Merge PR #400 from branch 'bwf-mixed-handoff'