RPC,GUI: support for long requests

CORE+RPC

- PerunRequest object now contains callbackId, which is sent with every
  callback from GUI (it is timestamp). It's used to address each unique callback.

- Removed sessionId from PerunRequest, since it was the same for all requests
  on one log-in. It was not logged or used anywhere and we couldn't address
  specific request by it.

- Pending requests are now stored in ConcurrentSkipListMap to allow simpler
  addressing by their callbackId.

- GetPendingRequests can send "callbackId" to get specific pending request.
  If parameter is missing, then list of all pending requests is sent back.

- When returning PerunRequest, JSONP serializer ignores perunPrincipal parameter.

GUI

- POST calls longer than 5 minutes display warning about pending request.
  Action in UI continues once all calls are resolved or user can simply close
  the warning confirm.

- GetPendingRequests checks for concrete request, not all and ignores any
  error in order to prevent bloating screen with error messages.

- Updated PerunRequest to match core version.