ship a blacklist of plugins in bzrlib

[Martin Pool]

We should put a list of (plugin, min_version, max_version) tuples into bzrlib so that we can blacklist plugins that don't work.

This should allow plugins to be more confident that when run with future versions of bzrlib they will either work or fail cleanly. They should therefore have less need to insist on a particular version of bzrlib. If it turns out that we do break a particular plugin and can't revert the change, we can add a blacklist entry before bzr's released.