Bugfix reading configuration on windows

Fixed bug where windows path seperators caused ReadAuthenticatedFile to fail looking up a file in the index.

Further refined feedback functions for pinchange request.

When parsing irma_configuration, we now check the timestamp of each scheme manager to decide if that manager must be copied out of the assets into storage, instead of the (scheme manager-transcending) file irma_configuration/timestamp.

Attributes may now also remotely be made optional.

Before 42a3acc2, more than one instance of the singleton credential type may have been stored.

David pinchange