Reduce booting time by using is_empty method

[HoOngEe]

The previous version of codechain booting time is so slow when the db is heavy. The reason is that when codechain checks if db is empty, it used db.keys().is_empty(). The routine had collected all the keys in the db to a hashmap, so the majority of the booting time was occupied by the routine for iterating and collecting keys.
I sent the method is_empty in JournalDB up to HashDB and used the method in db empty checking.

It closes #1513

[foriequal0]

Why do we have method that returns entire keys for the DB in the first place?

[HoOngEe]

The original method keys() in HashDB is part of parity's implementation and I'm just guessing that we used db.keys().is_emtpy() because there was no db.is_empty().