* Move and rename BlockchainCodec, BlockchainCodecRequest.
These are not specifically "blockchain", rather they are
specific to how gorilla/rpc works.
* Move claimtrie-related service/handlers to jsonrpc_claimtrie.go.
* Pull out decode logic into named func newBlockHeaderElectrum().
* Rename BlockchainService -> BlockchainBlockService.
* Drop http.Request arg from handlers, and use RegisterTCPService().
* Implement GetStatus() to pull data from HashXStatus table.
* Make the service objects independent, so we don't have inheritance.
* Add core session/subscription logic (session.go).
Implement subsribe/unsubscribe handlers.
* Support both pure JSON and JSON-over-HTTP services.
Forward NotifierChan messages to sessionManager.
* Only assign default port (50001) if neither --json-rpc-port nor
--json-rpc-http-port are specified.
* Handle failures with goto instead of break. Update error logging.
* Add --max-sessions, --session-timeout args. Enforce max sessions.
* Changes to make session.go testable. Conn created with Pipe()
used in testing has no unique Addr.
* Add tests for headers, headers.subscribe, address.subscribe.
* HashXStatus, HashXMempoolStatus not populated by default. Fix GetStatus().
* Use time.Ticker object to drive management activity.
