Introduce Groups Feature (#107)

Groups allows more control over swapping behaviour when a model is requested. The new groups feature provides three ways to control swapping: within the group, swapping out other groups or keep the models in the group loaded persistently (never swapped out). 

Closes #96, #99 and #106.

proxy/process.go

@@ -301,6 +301,10 @@ func (p *Process) start() error {
 }
 
 func (p *Process) Stop() {
+	if !isValidTransition(p.CurrentState(), StateStopping) {
+		return
+	}
+
 	// wait for any inflight requests before proceeding
 	p.inFlightRequests.Wait()
 	p.proxyLogger.Debugf("<%s> Stopping process", p.ID)