Use bytes as snapshot id and get rid of dict manipulation in check_snapshot()
This would break tests in other swh-loader-xx unless swh-loader-mercurial!28 (closed) swh-loader-svn!42 (closed) and swh-loader-git!46 (closed) are landed. <- this is possibly a lie...
In fact it would break loader git and mercurial until D3516 and D3517 are landed.
Migrated from D3501 (view on Phabricator)