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.
I believe it is safe to go (it is also part of my refactoring). But before committing the code, I still need to verify the scheme works for a number of occasions.