EndpointServerProvider with next() and random() for #212

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Implement random round-robin for registration (#212)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Hold/release message center from the activity

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Implementation of random round-robin for registration server selection

This closes #217.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Merge pull request #217 from kontalk/registration_server

Use a registration-enabled server