Distributed lock

[jesty]

I'm using Spring Statemachine in a microservice cluster, so I have to face with concurrent access to a state machine from various microservice instances.

I can't use Zookeeper, so I decide to use a distributed lock using Redis.

In the pull request I implemented it using two open-source projects: Redisson and ShedLock.

Using ShedLock you can relay also on other systems like a SQL database and MongoDB.

To use it you have to configure a Guard where you would like to have the lock:

               transitions
                    .withExternal()
                    .guard(new LockStateMachineGuard<>(lockService, 120))
                    .source("S1").target("S2")
                    .event("E1")

To remove the lock when the transition ends you have to configure a listener:

              config
                    .withConfiguration()
                    .listener(lockListener)
                    .autoStartup(true);

I tried to implements it using a StateMachineInterceptor, but it doesn't work.

What do you think about the pull request? Can it be useful? If you would like to merge I can contribute also to documentation.