Fix crash when updating a storage context with active requests

Requests keep pointers to objects owneded by the storage context such
as the SSL context handler and HTTP cache. This means we must stop
all requests before destroying the context, otherwise the requests
will access deleted data.

Change-Id: I67863287b3ffb470c8d84ff6cac143493998a6da
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>