RFR: 8283335 : Add exists and readAttributesIfExists methods to FileSystemProvider

[Lance Andersen]

Hi,

Please review the following patch which will:

- Enhance the java.nio.file.spi.FileSystemProvider abstract class to include the methods

  - public boolean exists(Path path, LinkOption... options)
  - public <A extends BasicFileAttributes> A readAttributesIfExists(Path path, Class<A> type, LinkOption... options)


This change allows for providers to provide optimizations when the file's attributes are not needed.

Mach5 tiers 1 - 3  run clean with this change

The CSR may be viewed at [JDK-8283336](https://bugs.openjdk.org/browse/JDK-8283336)


Best,
Lance

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Commit messages:
 - Add exists and readAttributesIfExists methods to FileSystemProvider

Changes: https://git.openjdk.org/jdk/pull/9249/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9249&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283335
  Stats: 342 lines in 10 files changed: 177 ins; 129 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/9249.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9249/head:pull/9249

PR: https://git.openjdk.org/jdk/pull/9249