add a new comparator sorted by keys of user view

[shenxingwuying]

Feature Request

Is your feature request related to a problem? Please describe:

scan with prefix filter very slow，and the rocksdb‘s order of key（rocksdb’s key） is not the same with the order of user key.
coz rocksdb‘s key sorted by bytes order and bytes with a length。And because of the order is not sorted by user key, the scan interface's semantics is not same with hbase and others system.
Maybe someone will have wrong understanding and use the scan interface with wrong understanding.

Describe the feature you'd like:
I will add a new customized Comparator, which will sorted by user key.

Describe alternatives you've considered:
scan timeout problem， see the discussion at : #723

Teachability, Documentation, Adoption, Migration Strategy:

I will support an new option, which will make sure. new cluster suggest use the new Comparator. but If old cluster has data, the comparator not change.