external API for the uploader to scan for changes in the filesystem

[vu3rdd]

@meejah and I were chatting about Uploader and were thinking that uploader could benefit from having an explicit API to scan the filesystem for changes which could be useful for testing and for external applications like Gridsync to use.

[meejah]

Further discussion about existing user-research indicates we might also want a mode where the only way new snapshots appear is via API requests (which would be triggered by a UI/UX tool probably via user action). That could be this API. So maybe we want the API to be "check for changes to: [file0, file1]" so that a frontend tool can offer the user the chance to make new snapshots for just a single file. If the file-list is empty we could scan everything.

[meejah]

Note that the latter API exists ("make a Snapshot for this one file") and there's a PR for a CLI command to trigger it (#309)

[meejah]

There is an HTTP API to do both a local scan or a remote poll