LocalIndexedRepo: generate a checksum file for the index

It uses SHA256 and is called the same as the index file, with a
'.sha' extension.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
CachingUriResourceHandle: use AbstractIndexedRepo.REPO_INDEX_SHA_EXTENSION

Instead of hard-coding it.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Add some comments

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>