Optimize conversion of byte arrays back to path string

DFSUtil.byteArray2PathString generates excessive object allocation.

1. each byte array is encoded to a string (copy)
2. string appended to a builder which extracts the chars from the intermediate string (copy) and adds to its own char array
3. builder's char array is re-alloced if over 16 chars (copy)
4. builder's toString creates another string (copy)

Instead of allocating all these objects and performing multiple byte/char encoding/decoding conversions, the byte array can be built in-place with a single final conversion to a string.