Enable TaskMonitor warnings of slow tasks by default #278

HBase has a TaskMonitor that can warn on any tasks that take too long.
https://github.com/apache/hbase/blob/47996d6c2128815e45bb8bdb6e3a470bfddd6106/hbase-server/src/main/java/org/apache/hadoop/hbase/monitoring/TaskMonitor.java#L49

By default it does not warn as the setting hbase.taskmonitor.rpc.warn.time defaults to 0 which disables the check. But it's definitely useful to have a warning like this.

We saw tasks stuck for a week at a customer and this went unnoticed. As I'm not 100% sure on which tasks (i.e. also Procedures?) are monitored I suggest a rather high threshold.
Maybe 1 hour? (Even though most tasks should finish within seconds)

This is a warning that we then - later - should extract from the logs to convert into an alert and/or at least have a dashboard (in Grafana or Opensearch Dashboard) to show these.
The log message will start with: "Task may be stuck"

Also note that older versions have a bug that can wrong warnings: https://issues.apache.org/jira/browse/HBASE-22935