Suggestion: Check portion of files

[marquis-de-muesli]

Right now the check command either only verifies the existence of chunks, or verifies hashes of all the chunks at once.
It might be useful to have an intermediate option which allows for verifying a custom amount of data (duplicati has similar).
B2 for example has 1G of free download each day, so you could schedule a free daily fractional verification.
Easiest would be just select samples at random, or best keep track of when which chunk was verified and cycle through.