Better Property handling to deal with deprecation and variable substitution of Hadoop config

Traditionally Pig handles hadoop configuration using PigContext.properties, the flow is:
1. Instantiate Hadoop Configuration, read all entries and save into PigContext.properties
2. adding system properties, pig.properties and "set" command in script into PigContext.properties
3. Every time we need to instantiate a Hadoop Configuration, we iterate PigContext.properties and add to Hadoop Configuration

This approach does not deal with hadoop 23 deprecated config option. Eg, in hadoop 23, "mapred.output.compression.codec" is replaced with "mapreduce.output.fileoutputformat.compression.codec". mapred-default.xml contains "mapreduce.output.fileoutputformat.compression.codec=org.apache.hadoop.io.compress.DefaultCodec". In Pig script, user may override it with "set mapred.output.compression.codec 'org.apache.hadoop.io.compress.BZip2Codec'". This is what happen:
1. Pig instantiate Hadoop Configuration, and put mapreduce.output.fileoutputformat.compression.codec=org.apache.hadoop.io.compress.DefaultCodec into PigContext.properties
2. Adding "mapred.output.compression.codec=org.apache.hadoop.io.compress.BZip2Codec" to PigContext.properties
3. When creating Hadoop Configuration to submit Hadoop job, Pig iterate PigContext.properties, it first see "mapred.output.compression.codec=org.apache.hadoop.io.compress.BZip2Codec", Hadoop Configuration translate it into the new property "mapreduce.output.fileoutputformat.compression.codec=org.apache.hadoop.io.compress.BZip2Codec", which is right until this point. Then Pig see "mapreduce.output.fileoutputformat.compression.codec=org.apache.hadoop.io.compress.DefaultCodec", and overwrite the previous right entry.

In PIG-2508, we address the issue by using a Configuration to handle system properties, pig.properties and "set" command, the flow is:
1. Instantiate Hadoop Configuration, adding system properties, pig.properties, then read all entries and save into PigContext.properties
2. For every set command, instantiate Hadoop Configuration with PigContext.properties, set the property to Configuration (Configuration translate the old option into new option), then read all entries back into PigContext.properies

This works but is cumbersome when doing "set".

In trunk, I want to use the following approach:
1. Write a subclass PigProperties extends Properties, and use it as PigContext.properties. The interface for PigContext remains the same
2. In PigProperties, we maintain a set of hadoop properties, a set of system properties, a set of pig.properties and a set of "set" command properties
3. Upon invoking PigContext.getProperties(), we instantiate hadoop configuration, put all properties in a sequence, then flatten it into a combined properties
4. We can do optimization to avoid recreating combined properties every time we call PigContext.getProperties()

The benefit for this approach:
1. Solve deprecate Hadoop config in a more clear way
2. Separate different layer of properties to ease the debugging, also provide potential to show properties to the user at different level
3. Potential to add job level properties in the future
4. No backward incompatibility introduced