Avoid obtaining any locks when creating/sending heartbeats

When NiFi creates a heartbeat to send to the coordinator, it must obtain a few locks in order to generate that heartbeat. We should avoid obtaining any read locks, write locks, or synchronized monitors, especially those that may be held for a while. Doing so can result in NiFi getting disconnected from the cluster if a write lock is held for a long time.

Specifically, the following locks are obtained, at minimum:

• FlowController readLock in the createHeartbeatMessage() method. Due to refactoring, this read lock is not necessary at all.
• revisionManager.getRevisionUpdateCount() is synchronized. However, the synchronization here is not needed, as it just returns an AtomicLong.get(). This is perhaps the most important lock to avoid because any update to a component or group of components happens within revisionManager.updateRevision, which also is synchronized. So a large request like deleting thousands of components will block heartbeats from being created until this completes.
• FlowController.getTotalFlowFileCount - this may be the most challenging to eliminate. It calls ProcessGroup.getConnections() and ProcessGroup.getProcessGroups(), which means that it must obtain the read lock of the Process Group twice - for every Process Group in the flow. We may be able to change StandardProcessGroup's connections and processGroups maps to ConcurrentHashMap's and just introduce a getQueueSize() method on ProcessGroup that can avoid having to lock so much
• This createHeartbeatMessage() method also appears to reference FlowController's connectionStatus member variable without any locks, although it is not volatile and documentation indicates that it's guarded by read/write lock. So that needs to be addressed in order to ensure that the connectionStatus is always accurately reported.