feat(tiller): limit number of versions stored per release

This adds a new configuration option to Tiller to limit the number of
records stored per release.

Tiller stores historical release information (helm history, helm
rollback). This makes it possible to set a maximum number of versions
per release.

To enable this feature, use `helm init --history-max NNN`. Note that
because of the restrictions on Deployment objects, you will have to
re-install Tiller to add a limit.

Along the way, I found an unreported bug in the Memory storage driver.
This fixes that bug and adds substantially more tests to catch
regressions.

Closes #2332