Save pending local transactions in the database (#4566)

* Create new column family for local node info

* remove DBTransaction::new reliance on DB

* KeyValueDB trait

* InMemory KeyValueDB implementation

* journaldb generic over KVDB

* make most of `ethcore` generic over KVDB

* fix json tests compilation

* get all tests compiling

* implement local store (just for transactions)

* finish local store API, test

* put everything into place

* better test for skipping bad transactions

* fix warning

* update local store every 15 minutes

* remove superfluous `{}`s