[FOLD] Refactor structure of Server and ServerHandler sources:
* Rename http module to server module
* Rename onMaybeMove to onHandoff
* Merge HTTP::Server and ServerHandler into the server module
* Split classes to individual headers to reduce type exposure in includes
* Move some JSON-RPC utilities into the server module
Change to the Application io_service:
* Simplified the implementation and removed class IoServicePool
* The io_service outlives the components of the Application