The current design of the API is that all questions are answered in-
order. This implies that handling of one connection will also just use
one thread.
If we want to improve performance of things like "sendTransaction" we
want an async option, which runs the action and generates the answer
when its done in a separate thread (a new one for each async-request).
This breaks the in-order reply sequence for clients and as such it
requires them to set a new header-option to enable this.
TomZ