Merge pull request #174 from whirm/improve_candidate_updates

Replace candidate instead of setting a new sock_addr, this way the
Let Callback.call warn on timeout.
Add a unit-test description.
A DELAY parameter for Callback.call makes no sense
Merge pull request #175 from boudewijn-tribler/call-warning

Let Callback.call warn on timeout.
Fixes duplicate callback identifier bug.

Using RequestCache identifiers (which are unique per community) for
Callback identifiers (which should be unique per Dispersy) will
obviously cause conflicts.
Merge pull request #176 from boudewijn-tribler/request-cache-identifier

Fixes duplicate callback identifier bug.
Allow to pass a candidate list.
Also add tests for this feature.