Fix memory leak in health check

1. On connection disconnect, health check loop ends immediately
   and independently on probe interval setup
2. Proper resource cleanup

On disconnect, health check terminates itself with a delay based
on the probe interval. It causes memory usage going up if
AsyncConnect() & Disconnect() are called in a loop.

Signed-off-by: Vladimir Lavor <vlavor@cisco.com>
Change-Id: Ibcdfec9d2d7736f9390e21039d9a917228114273