Introduce isChecksumPillar on MessageResponses in general

Currently the protocol exposes a PillarChecksumSpec attribute on the operations which can deliver checksums in responses. This is a indirect way of telling the client that it is a checksum pillar. Only a single ChecksumSpec can be returned. This is always the default checksum for the reference checksum pillar. It would be much simplere to just return a boolean, indication this is a checksum pillar.

This would also make it clearer in the GetFile operation what is going on.

The original idea was to only make the presence of checksum pillars visible to clients through it's capabilities in the responses, but this leads to somewhat awkward code to handle the special case for checksum pillars.