Better bytes skipping in FsBlobContainer

[tlrx]

This pull request is addressing a // NORELEASE tag in FsBlobContainer where the InputStream opened by the readBlob(String) does not efficiently skip bytes under hood.

This method uses the Files.newInputStream(resolvedPath) which uses the Files.newByteChannel() which returns a SeekableByteChannel instance that allows to efficiently skip bytes but sadly this channel is then wrapped using Channels.newInputStream() into a ChannelInputStream instance that delegates the skip() method execution to the default InputStream implementation which reads bytes.

This pull request changes the implementation of the readBlob(String, long, long) to use a more effective manner to skip bytes and adds a test to verify that only the required number of bytes are read. It also swaps how the buffered and limit streams are combined so that the underlying stream is limited first and the buffered stream will not try to buffer more bytes than necessary.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)

[tlrx]

@elasticmachine update branch

[tlrx]

@elasticmachine update branch

[DaveCTurner]

LGTM. Taught me about a whole other layer of mocking that I didn't know about yet :)

[tlrx]

Thanks David

Taught me about a whole other layer of mocking that I didn't know about yet :)

Like an onion: the more you peel it the more you cry :)