RFR: 8319531: FileServerHandler::discardRequestBody could be improved [v3]

[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

Darragh Clarke has updated the pull request incrementally with one additional commit since the last revision:

  updated drain to use the new skip method

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16616/files
  - new: https://git.openjdk.org/jdk/pull/16616/files/8ebf6e09..0a7bdae4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16616&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16616&range=01-02

  Stats: 13 lines in 1 file changed: 0 ins; 9 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