Give personas a sturdyref

This change primarily is to give each persona a publically accessable
sturdyref that in normal circumstances should never change. This
sturdyref can be used by others to refer to this persona. This can be
accessed via a new 'get-sturdyref method on the persona itself.

Another important change that is included in this commit is the
'commit method on the persona-manager. It now returns a promise to
commit the items and items should not be considered committed until
the promise resolves. This is due to how sturdyrefs are made.

Additionally this adds a test to verify that the set-persona-name
method does as expected.