Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56521
Implement a TODO
Reuse the async write buffer between writes to reduce allocation and GC overhead. Based on a patch by leonzhx.
Also:
 - don't re-allocate the buffer / move the data in the buffer if the buffer is partially written on a subsequent write
 - make the buffer size configurable, defaulting to 8k

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1596275 13f79535-47bb-0310-9956-ffa450edef68