Fixed TACHYON-97

Current implementation of read(long position, byte[] buffer, int offset, int length) in HdfsFileInputStream is incomplete: 
1. It does not consider the boundary of file length, which could lead to an out of range issue when seeking the requested position in bytebuffer. 
2. It does not recover the original offset after function call.
If i remember correctly, isn't there a recursion issue if you set `fs.defaultFS` as tachyon?  So if core-site.xml points to tachyon as the default, wouldn't removing this cause issues?