Snapshot checksum verification

Adds automatic verification of all files that are being snapshotted. Closes #5593
@rmuir I added a test to verify that, but I should probably rename [readIndexInputFully](https://github.com/elasticsearch/elasticsearch/pull/7159/files#diff-0c966f9523342d88888d48b7e014ef58R404) into something like readIndexInputFullyWithRandomeSeeks to reflect that.