[UWS] Support the blocking behavior described in PR-UWS-1.1.

It is possible to choose how the blocking mechanism should behave
(e.g. what the max. waiting period, how many requests can be blocked
in the same time, what happen when the blocking times out, ...).

Indeed, the policy to apply must actually be an extension of the interface
BlockingPolicy. Already two implementations are provided in the library
(LimitedBlockingPolicy and UserLimitedBlockingPolicy), but a custom policy
can perfectly be created and apply to a UWS service.

By default, no policy is set. In such case, the service will block the time
specified by the user, which may be -1 (i.e. wait indefinitely). A
BlockingPolicy can help controlling the waiting/blocking process and protect
the resources of the server.