db refactor (#385)

* db refactor in progress, refactored kvdb

* kvdb refactor

* removed unused RwLock

* removed unused RwLock

* BlockChainDatabase

* db refactor in progress

* implemented BlockProvider for newdb

* fixed compiling

* newdb TransactionMeta

* newdb tests

* reopen tests for newdb

* replaced old db with new db

* chain_verifier uses new db

* sync module uses refactored db

* check double spends in verification layer

* added Forkable trait to satisfy resolving recursive types by compiler

* new db regtests in progress

* fixed verification and sync unit tests

* fixed miner and rpc unit tests

* implemented fmt::Debug for few structs

* removed commented out code

* fixed bencher target

* removing commented out code, polishing db_refactor pr