Replace all instances of rpcclient.ClientConnector with birpc.ClientConnector.
Pass context, maxReconnectInterval, delayFunc and birpcClient to rpcclient
constructors.
Remove redundant time.Duration conversions (e.g. time.Duration(1*time.Second)
now becomes time.Second.
Add context where needed (context.Background() for tests, context.TODO()
for places where it should be passed from somewhere else).
Implement that functionality of the SessionSv1.Sleep call, in sessions/sessions
instead of apier/v1.
Make changes in utils/server.go (replacing the old rpc2 library with github.com/cgrates/birpc).
Change the way we register birpc methods for sessions in services, using a helper function
defined in engine/libengine.go.
