Use homomorphic hash to checksum Replicas

Adrian (from Snowflake) [proposes](https://docs.google.com/document/d/1KXvlHncC9vunrXdW6kecwEcVEzL7uh07I6BBrdvOJ00/edit#) the use of a [homomorphic hash function open sourced by Facebook](https://engineering.fb.com/security/homomorphic-hashing/) to verify the consistency of partitions within FDB.

Currently we use the consistency checker to achieve this. However, in the case of a corruption it could potentially take the consistency checker a very long time to detect it. Furthermore this could also be used to verify the correctness of a backup and/or snapshot backup without comparing the whole database.

The above document discusses more details.

Read the linked design document and Facebook's homomorphic-hashing article first, then trace the existing consistency checker and the backup and snapshot verification paths. Determine how the proposed hash would cover partition consistency and verification without comparing the whole database; done means the design and implementation are validated for those use cases.

Written by the indexing model from the issue text.