[util] Handle thread exceptions

[wjgra]

Exceptions thrown by server/client threads are currently uncaught - consider a ThreadContext class that includes an atomic<bool> shouldExit to manage thread lifetime.

[wjgra]

Redundant in view of #10 - we can just use std::futures instead, which propagate exceptions upon calls to get().

[wjgra]

Re-opened: the ordering imposed by get() means that an exception being thrown in a given thread does not terminate the program. This is worth investigating further.