Reduce duplicate/unused string data from FileDescriptor/FileBlock thrift structs

Made the following changes:

1) Removed filePath from FileBlock. It wasn't used anywhere.
2) Update FileDescriptor to use file name rather than file path. The full path to the file
   can be found by prepending the parent partition directory the file name.
3) Removed fileLength from FileBlock. This was the same length used in FileDescriptor.

Testing these changes out on two table on a 20 node cluster. One table had ~100K blocks
and one table had ~15K blocks.

In both cases I saw the following improvements:

* After making change 1) the total serialized size of the table dropped by ~30%
* After making change 1) and 2) the total serialized size of the table dropped by ~52%
* After making change 1), 2), and 3) the total serialized size of the table dropped by ~55%

Change-Id: Ic85b3cbcf775569f69b7303bec4adc52593fc35c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1351
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins