Juwang-Inserting LoggingScopes

With LoggingScope and LoggingScopeFactory, this PR is to insert LoggingScope in REEF code at major stages to measure the performance. 
This is to resolve issue REEF-28 https://issues.apache.org/jira/i#browse/REEF-28?filter=-1

This shouldn't be checked in, right? Isn't it replaced with `LoggingScopeImpl`?