[scanner] add generic shard scanner

[mantas-sidlauskas]

What changed?
Shard scanner is a generic shard scanner.

To use shard scanner you need to provide:

• invariant manager - to set-up check and fix actions defined by scanner user.
• iterator - shard scanner uses iterator interface to iterate over entities.
• config resolver (optional) - scanner users can have their own custom dynamic configs. Config values are recorded for better determinism.

Why?
This shard scanner helps to reduce boilerplate when introducing new types of scanners, no need to reimplement/duplicate common logic.

How did you test it?
Unit tests.

Potential risks
None, this is noop change

[coveralls]

Coverage increased (+0.08%) to 59.52% when pulling 777a50b on mantas-sidlauskas:shard_scanner into 8055ae8 on uber:master.