ChunkedNioFile can use absolute FileChannel::read to read chunks

[franz1981]

Motivation:

Users can reuse the same FileChannel for different ChunkedNioFile
instances without being worried that FileChannel::position will be
changed concurrently by them.
In addition, FileChannel::read with absolute position allows to
use on *nix pread that is more efficient then fread.

Modifications:

Always use absolute FileChannel::read ops

Result:

Faster and more flexible uses of FileChannel for ChunkedNioFile

[netty-bot]

Can one of the admins verify this patch?

[franz1981]

I see that the original version was very naive (mine too) by changing FileChannel::position only when offset!=0.
If a user has already modified FileChannel::position and will call new ChunkedNioFile(in, 0, file.size, 8192), the old implementation was not changing FileChannel::position ie the next read would have started from offset == 0, but actually reading at FileChannel::position != 0.

[njhill]

LGTM

[normanmaurer]

@netty-bot test this please

[normanmaurer]

@netty-bot test this please

[normanmaurer]

@franz1981 thanks a lot!

[wy96f]

Good job :)