* [DEV-65] Renamed BestState to State and GetSnapshot to GetCurrentState.
* [DEV-65] Implemented State and SelectedTip, updated dagState (the serializable version of State), and updated State creations.
* [DEV-65] Fixed references to GetCurrentState and fixed tests.
* [DEV-65] Renamed State to DAGState, corrected comments, created a new thread-safe method setDAGState, fixed broken references I missed.
* [DEV-65] Converted dagState (the object) to dbDAGState, and made it serialize/deserialize into/out of JSON.
* [DEV-65] Renamed Txns to Txs and fixed a couple of comments.
* [DEV-65] Renamed some more "Txns" and removed some dead code.