changes to allow fixed checkpoint for files for underlayer fs

Patch for allowing users to specify if they want a fixed checkpoint for their file. By setting "tachyon.fixed.checkpoint" to be true, the Tachyon file will be written to "COMMON_CONF.UNDERFS_ADDRESS + info.path" where info.path is the substring "x" in "tachyon:master:port/x" that the user requests to write to. So if user wants to write to "tachyon://localhost:19998/user1/file.txt" and HDFS is the backing fs, then the file "hdfs://hdfsmaster:port/user1/file.txt" will be created. 

This is the checkpoint path reported to the master, so all further reads to this file will be handled appropriately. One issue though is that this is a system wide setting that could be toggled on or off by user at any point in time, which may lead to some confusion. Some of this confusion can be cleared up if we make the user specify PER file if they want fixed checkpoint or not. This is not that big of a deal though.