[keycloak-dev] Support concurrent startup by more cluster nodes

[Marek Posolda]

Send PR with added support for $subject . 
https://github.com/keycloak/keycloak/pull/2332 .

Few details:
- Added DBLockProvider, which handles acquire and release of DB lock. 
When lock is acquired, the cluster node2 needs to wait until node1 
release the lock

- The lock is acquired at startup for the migrating model (both model 
specific and generic migration), importing realms and adding initial 
admin user. So this can be done always just by one node at a time.

- The lock is implemented at DB level, so it works even if infinispan 
cluster is not correctly configured. For the JPA, I've added 
implementation, which is reusing liquibase DB locking with the bugfix, 
which prevented builtin liquibase lock to work correctly. I've added 
implementation for Mongo too.

- Added DBLockTest, which simulates 20 threads racing for acquire lock 
concurrently. It's passing with all databases.

- Default timeout for acquire lock is 900 seconds and the time for lock 
recheck is 2 seconds. So if node2 is not able to acquire lock within 900 
seconds, it fails to start. There is possibility to change in 
keycloak-server.json. Is 900 seconds too much? I was thinking about the 
case when there is some large realm file importing at startup.

Marek