Improve logging for lease management

[cgillum]

I was doing some manual testing of the lease management code to try and diagnose some issues we've been facing recently and found a few places where we could use some improvements. Here are the changes in this PR:

• Add "lease type" fields to all the lease management logs for easier analysis
• Fixes an ObjectDisposedException when restarting the task hub worker (reported by an internal DTFx user).
• A small number of minor code cleanup changes

I found some more fundamental lease management issues as well, but I'll try to tackle those in a separate PR.

[sebastianburckhardt]

some minor questions, other than that this looks good to me.

[davidmrdavid]

LGTM, although I'm not sure where the object-disposed exception was addressed.

[cgillum]

@davidmrdavid thanks! The ObjectDisposedException was in AppLeaseManager, where we try to reuse an old starterTokenSource, which had been previously disposed. My change was to assign it to null as part of shutdown so that we'd reinitialize it on the subsequent startup.