RFR: 8319531: FileServerHandler::discardRequestBody could be improved

[Darragh Clarke]

**Problem**
`discardRequestBody` calls `InputStream::readAllBytes` to read and discard all the request body bytes. This is somewhat wasteful as they can instead be skipped.

**Changes**
- Updated `FileServerHandler::discardRequestBody` to use `InputStream::skip`.
- Created skip in `LeftOverInputStream` based on [InputStream](https://github.com/openjdk/jdk/blob/c9657cad124d2be10b8d6006d0ca9a038b1c5945/src/java.base/share/classes/java/io/InputStream.java#L540). This gets used by `FixedLengthInputStream` and `ChunkedInputStream` which previously had been using the skip implementation from `FilteredInputStream` which would have caused blocking.

- Also made a minor change to `LeftOverInputStream::Drain` to change bufSize.


I ran test tiers 1-3 and all tests are passing with these changes

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Commit messages:
 - added a skip method for leftOverInputStream and altered drain method

Changes: https://git.openjdk.org/jdk/pull/16616/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16616&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319531
  Stats: 30 lines in 2 files changed: 25 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16616.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16616/head:pull/16616

PR: https://git.openjdk.org/jdk/pull/16616