Fix proxycommand leaks

[greschd]

Fixes #2018 and fixes #1923 and fixes #1853

This addresses two issues:

• The constructor of _DetachedProxyCommand should not call super. The reason for this is that the detached version creates a different subprocess. If the super is also called, two subprocesses are created.
• In the close method of _DetachedProxyCommand, the subprocess is killed more aggressively, and we poll the subprocess to avoid having defunct processes.

The subprocess killing is implemented as follows:

• .terminate() the subprocess
• .poll() for 2 seconds to see if it exits
• if it hasn't, .kill(), and .poll() again for 2 seconds

This part is a bit hand-wavy, but I wanted to avoid using the .wait() method. On balance I think it's better if the daemon just continues if the subprocess is really unkillable.

[coveralls]

Coverage increased (+4.5%) to 67.692% when pulling 613e2a7 on greschd:fix_proxycommand_leaks into f9a5624 on aiidateam:develop.

[sphuber]

Thanks @greschd