Destroy unit's machine if eligible.

Either way, assert that the state assumptions for being eligible or
not-eligible hold true for the transaction.
Improved state assertion in removeUnitOps

More test cases, renamed destroyHostOps
destroyHostOps fix, allow machine removal on retry

If a unit's host cannot be removed, make that condition the only
assertion on the transaction, so that we can re-evaluate the state
completely on retry. Examples:
- host container removed after check, before txn exec
- host gives up voting rights after check, before txn exec

Unit tests added for the above, and the converse (host qualifies on
check, disqualified before txn exec).

Fix import
Add thrashing state testcase.
Consolidate destroyHostOps machine ops.

$or together all the "keep host alive" assertions, so that a transaction
can succeed if the individual "keep" criteria change in a transaction
but the outcome remains the same.
Improve destroyHostOps test cases.

Test coverage for removing an unassigned unit. Use assertLife helper
function. More descriptive test names. More assertion checks.
Improve non-empty containers assertion.

MongoDB makes it kind of weird.
https://stackoverflow.com/questions/14789684/find-mongodb-records-where-array-field-is-not-empty-using-mongoose
Clean up tests, log removal of unassigned unit.
Removed test cruft, .Refresh() from txn hooks.

Colocated unit set up outside of the transaction hook in
TestRemoveUnitMachineRetryOrCond.
Remove TestDestroyCleanRelations.

Check unit destroyed in TestRemoveUnitMachineRetryOrCond
Consolidate & improve destroyHostOps assert conds.

Refactor out unit test suite method setMachineVote.  More comments.
Remove subordinate unit test case.
Merge remote-tracking branch 'upstream/master' into rm-unit-machine
Updated wrt state/txn refactoring.

Fixed service removal test by calling State.Cleanup.