- Support stop cluster mode (`NOT-STARTED` mode)
- Fix bug when updating cluster state via command API (should modify via state property)

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Code refinement for heartbeat component in transport related module
- Add remove support of `SentinelConfig`

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Modify the init order of command center init func to -1 (prioritized than before)
- Add CommandConstants for common use

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Carpenter Lee <hooleeucas@163.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Add several command handlers
- Update cluster state manager

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

normalize metric fields and methods:
- rename exception to exceptionQps;
- remove ed postfix of all fields and methods about metrics;

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Add `close` method in WritableDataSource (to extend AutoCloseable in JDK 1.7 later)
- Separate the writable file data source from original class
- Add a sample to show how to register data sources via Sentinel init mechanism
- Separate a writable data source registry from original handler to make it clear

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Refine code about writable datasource

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Sentinel should not send heartbeat to dashboard until the runtime port has been settled down

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Use named thread factory with all thread pools for identification

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>