Make currency conversion rate updates explicit and async

Previously this happened implicitly on the first currency conversion
needing an update, via a sub-eventloop and with various attempts to
prevent reentrancy issues (with limited successes, QML managed to bypass
all that and deadlock).

A simple conversion API should however neither trigger implicit network
operations nor should it have unexpected and unpredictable blocking or
locking behavior. Therefore extend the existing API to trigger updates
to return a simple job object for (optional) notification of completion,
and require consumers using currency conversions to call that periodically.