storage server deletes garbage shares itself instead of waiting for crawler to notice them

[zooko]

Currently, the "lease crawler" or "accounting crawler" is responsible for deleting shares that have lost all their leases (by cancellation or expiry).

I propose that this be done instead by the storage server maintaining a persistent set of shares to be deleted. When lease-updating step (which, in #666, is synchronous and fast) has identified a share that has no more leases, the share's id gets added to the persistent set of shares to delete. A long-running, persistent, duty-cycle-limited processes deletes those shares from the backend and removes their ids from the set of shares-to-delete. This is cleaner and more efficient than using a crawler, which has to visit all shares and which never stops twitching, since this has to visit only shares that have been marked as to-delete, and it quiesces when there is nothing to delete.

This is part of an "overarching ticket" to eliminate most uses of crawler — ticket #1834.