[remote/botostore][s]: set _checksum based on etag for boto backends (was not available if we had not set in 'our' metadata on the way in).
[remote/botostore][s]: set _checksum based on etag for boto backends (was not available if we had not set in 'our' metadata on the way in).

* TODO: no test for this case as our tests do set the checksum on way in.

okfn/ofs