Make metrics naming consistent

While working HADOOP-6728, I noticed that our metrics naming style is all over the place:

• Capitalized camel case: e.g., "FilesCreated" in namenode metrics and some rpc metrics
• uncapitalized camel case: e.g, "threadsBlocked" in jvm metrics and some rpc metrics
• lowercased underscored: e.g., "bytes_written" in datanode metrics and mapreduce metrics

Let's make them consistent. How about uncapitalized camel case? My main reason for the camel case: some backends have limits on the name length and underscore is wasteful.

Once we have a consistent naming style we can do:
@Metric("Number of INodes created") MutableCounterLong filesCreated;

instead of the more redundant:
@Metric(

) MutableCounterLong filesCreated;