ethcore-db crate (#1097)

* trait

* implentated, lifetime issue still

* full api

* test mod

* working open

* get/retrieve

* fix warnings and bug

* working serialization of &[u8] parameters

* client attributes

* fix empty payload ser/de

* [ci skip] debug assert out

* extra deserialization test

* extra serialization test

* extra serialization test

* serialization fixes, nupdate rocksdb

* open test working

* result bug & remove some scaffolds

* fix warnings

* more simple tests

* consistent quotes

* get rid of dedicated is_open flag

* hashmap -> btreemap