Adds SshjTool exec script async
ShellToolAbstractTset.execCommands: remove duplication
ssh-async: use long-polling

- Polling ssh command executes until either command completes,
  process no longer running, or timeout.
- Adds PROP_EXEC_TIMEOUT for control when ssh commands should timeout.
- Adds PROP_EXEC_ASYNC_POLLING_TIMEOUT for the long-polling of
  async exec, to timeout each poll.
install java: use async-ssh-exec
ssh async-exec: incorporate comments + logging
This closes #358