Add bidirectional support for sessions while maintaining changing the
current rpc service registration method. Modified methods in
sessionsbirpc.go file to satisfy the birpc.ClientConnector interface
and removed BiRPC prefix before creating the service that's to be
registered.
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.
