BUG-1949 Fix race condition in AsyncSshHandler

Remove pending limit due to unexpected behaviour with chunked messages.
Extract Reader/Writer into separate classes.

Also lower the amount of requests sent in SSH Stress integration test.

Change-Id: Idff719ac3a6bed9e8939efa01b8306f2884848fe
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>