RFR: JDK-8273038: ChannelInputStream.transferTo() uses FileChannel.transferTo(FileChannel) [v6]

[Markus KARG]

> As proposed in JDK-8265891, this PR provides an implementation for `Channels.newInputStream().transferTo()` which provide superior performance compared to the current implementation. This PR is a spin-off from https://github.com/openjdk/jdk/pull/4263 and https://github.com/openjdk/jdk/pull/5179 and deliberately concentrates **only** on the case where both, source **and** target, are actually `FileChannel`s. Other types of channels will be discussed in future PRs.
> 
> * Prevents transfers through the JVM heap as much as possibly by offloading to deeper levels via NIO, hence allowing the operating system to optimize the transfer.
> 
> Using JMH I have benchmarked both, the original implementation and this implementation, and (depending on the used hardware and use case) performance change was approx. doubled performance. A rather similar approach in different use casse was recently implemented by https://github.com/openjdk/jdk/pull/5097 which was reported by even provide 5x performance (https://github.com/openjdk/jdk/pull/5097#issuecomment-897271997).

Markus KARG has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Testing random starting point within source and target
  
  Signed-off-by: Markus Karg <markus at headcrashing.eu>

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5209/files
  - new: https://git.openjdk.java.net/jdk/pull/5209/files/e9163bf4..a043ae25

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5209&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5209&range=04-05

  Stats: 13 lines in 1 file changed: 4 ins; 1 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5209.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5209/head:pull/5209

PR: https://git.openjdk.java.net/jdk/pull/5209