Add support for generating incremental streams when pushing volumes

This partially addresses #46 - but does not completely resolve it.

Added here is support for a new parameter to `Filesystem.reader` to specifying which snapshots the receiver has.  The implementation uses this to generate an incremental stream if possible.  Also added is `IFilesystem.snapshots` which can be used to get a value to pass for this parameter.

Work to glue this together with the actual push implementation will come in a follow-up.
Closest to the end of which supplied list? eg `[a, b, c, d], [b, c, d, e, f]` Does it make any difference?