`hbase.hstore.flusher.count` setting to 0 breaks HMaster

Summary 

The configuration parameter `hbase.hstore.flusher.count` allows `0` (which turns out to be a bad value after I understand the source code).

When we set `hbase.hstore.flusher.count` to 0, HMaster will fail in 200,000 ms without any error message, but

```
java.lang.RuntimeException: Master not initialized after 200000ms

```

There’s no messages pinpointing `hbase.hstore.flusher.count` being the root cause.

0 means `MemStoreFlusher` is not alive. During the initialization time of HMaster, `isHealthy()` will be called and it will try to check if `MemStoreFlusher` is alive or not.  However, since we set `hbase.hstore.flusher.count` to 0, there will be no flusher created at the first place, which cause`isHealthy()` to return false. As a consequence, HRegionServer does not enter its main loop.

How To Reproduce 

The version of HBase: 2.2.3

1. Set the value of configuration parameter `hbase.hstore.flusher.count` to `0` in “conf/hbase-site.xml”
2. Start hbase by running “bin/start-hbase.sh”
3. Wait for 3-4 minutes
4. Open the log file “logs/hbase-<username>- master -<pcname>.log”