Change dfs.namenode.startup.delay.block.deletion to second instead of millisecond

Based on the discussion in https://issues.apache.org/jira/browse/HDFS-6186, the delay will be at least 30 minutes or even hours. it's not very user friendly to use milliseconds when it's likely measured in hours.
I suggest to make the following change
1. change the unit of this config to second
2. rename the config key from "dfs.namenode.startup.delay.block.deletion.ms" to "dfs.namenode.startup.delay.block.deletion.sec"
3. add the default value to hdfs-default.xml, what's the reasonable value, 30 minutes, one hour?