RFR: 4926314: Optimize Reader.read(CharBuffer) [v2]

[Philippe Marschall]

> Implement three optimiztations for Reader.read(CharBuffer)
> 
> * Add a code path for heap buffers in Reader#read to use the backing array instead of allocating a new one.
> * Change the code path for direct buffers in Reader#read to limit the intermediate allocation to `TRANSFER_BUFFER_SIZE`.
> * Implement `InputStreamReader#read(CharBuffer)` and delegate to `StreamDecoder`.
> * Implement `StreamDecoder#read(CharBuffer)` and avoid buffer allocation.

Philippe Marschall has updated the pull request incrementally with one additional commit since the last revision:

  Fix off-heap code path
  
  The off-heap code path was missing a buffer.put

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1915/files
  - new: https://git.openjdk.java.net/jdk/pull/1915/files/967b314a..a88cd931

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1915&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1915&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1915.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1915/head:pull/1915

PR: https://git.openjdk.java.net/jdk/pull/1915