Support manual rescan/sync for magic-folder

[cypher]

As it currently stands, after the initial startup scan/check, magic-folder will check for and download remote files according to the schedule specified by the poll_interval setting in tahoe.cfg. It may be useful, however, for users and applications to be able to check for remote changes on a "manual" basis in conjunction with or independently of the schedule specified by poll_interval. For example, if Alice and Bob belong to the same magic-folder and Alice informs Bob that she just uploaded a new file for him, Bob must wait some period of time (up to 60 seconds by default) or restart his tahoe client before the download proceeds (which may test Bob's patience). It may thus be useful for Bob (or any future magic-folder-based applications) to be able to make a some API call and/or run some subcommand (e.g., tahoe magic-folder rescan or tahoe magic-folder sync) to poll for remote changes immediately and download any new files that might be available.

[exarkun]

Push notification of change events from the grid to Bob's client would let Bob's client know about changes immediately and automatically.