multi file input format for loaders

Feature: combine splits of sizes smaller than the value of property "pig.maxCombinedSplitSize" or, if the property of "pig.maxCombinedSplitSize" is not set, the file system default block size of the load's location. This feature can be turned off through setting the property "pig.splitCombination" to "false". When such a combination is performed, a log message like "Total input paths (combined) to process : 7" will be logged. This feature will be applicable if a user input, or an intermediate input, has many small files to be loaded that would otherwise cause many more "under-fed" mappers to be launched and potentially slowdown of the execution. This change will not cause any backward compatibility issue except if a loader implementation makes use of the PigSplit object passed through the prepareToRead method where a rebuild of the loader might be necessary as PigSplit's definition has been modified. However, currently we know of no external use of the object. This change also requires the loader to be stateless across the invocations to the prepareToRead method. That is, the method should reset any internal states that are not affected by the RecordReader argument. Otherwise, this feature should be disabled. In addition, if a loader implements IndexableLoadFunc, or implements OrderedLoadFunc and CollectableLoadFunc, its input splits won't be subject to possible combinations.