API: New "read" that checks checksum provided by client

[shaitan]

Problem

When client reads data by chunks it can get useless garbage in case if:

1. the key was overwritten in the middle of the read
2. replica used for reading became unavailable and client was switched to inconsistent replica

New features

• Add ability of reading to return checksum of the record from footer.
• Add new "read" that takes checksum and checks it before read.

It allows to escape sending garbage to clients and makes switching between replicas safer.

[bioothod]

Please also add comparison with timestamp and user flags, i.e. if client-provided timestamp/user-flags doesn't match one on the disk, return error.

Timestamp comparison can include relation operator, for example, if on-disk timestamp is newer than that in client request, read returns special error.