HA: Standy should exit safemode when resources are from low available

The NameNodeResourceMonitor automatically enters safemode when it detects that the resources are not suffcient. NNRM is only in ANN. If both ANN and SNN enter SM due to low resources, and later SNN's disk space is restored, SNN willl become ANN and ANN will become SNN. However, at this point, SNN will not exit the SM, even if the disk is recovered.

Consider the following scenario:

• Initially, nn-1 is active and nn-2 is standby. The insufficient resources of both nn-1 and nn-2 in dfs.namenode.name.dir, the NameNodeResourceMonitor detects the resource issue and puts nn01 into safemode.
• At this point, nn-1 is in safemode (ON) and active, while nn-2 is in safemode (OFF) and standby.
• After a period of time, the resources in nn-2's dfs.namenode.name.dir recover, triggering failover.
• Now, nn-1 is in safe mode (ON) and standby, while nn-2 is in safe mode (OFF) and active.
• Afterward, the resources in nn-1's dfs.namenode.name.dir recover.
• However, since nn-1 is standby but in safemode (ON), it unable to exit safe mode automatically.

There are two possible ways fix this issues:

1. If SNN is detected to be in SM(because low resource), it will exit.
2. Or we already have HDFS-17231, we can revert HDFS-2914. Bringing NNRM back to SNN.