Several ref doc fixes

Issue: SPR-8182
Issue: SPR-11243
Issue: SPR-11292
Issue: SPR-11318
Fixed version number in xsd comment
Declared JDK 6 update 18 as minimum requirement
Mentioned "-parameters" compiler flag in javadoc
Fixed accidental use of JDK 1.7+ Integer/Long.compare methods

Issue: SPR-11319
Polishing
Introduced customizeConnection callbacks for URLConnection used by exists() / contentLength() / lastModified()

Issue: SPR-11320
Polishing
Write prelude on successive SockJS streaming requests

sockjs-client expects a prelude to be written on every request with
streaming transports. The protocol tests don't make this clear and
don't expose this issue.

The test case for SPR-11183 (writing 20K messages in succession) did
expose the issue and this commit addresses it.

Issue: SPR-11183