Removed `println` and fixed some XML errors.
Made the async SOAP client more generic.

It is now ready to be used for the NSI requester role.
Removed unused `connectionId` column from `NsiV2Message`.
Consistently pass a `Reservation` to `NsiConnectionProviderClient`.
Removed duplicate connection state update logic.

Removed additional indirection layer for most messages received from the
provider.
Remove reserve timeout poller.

The provider is now responsible for switching to the reserve timeout
state and notifying us. Since we're also a provider we also switch to
reserve timeout (unlike the aggregator).
Removed indirection layer when querying connections.
Fix warnings and removed unused code.
Made `AsyncSoapClient` usable with any NSI service.
Switched from Guava `Optional` to Java `Optional`.