SSH: Close unused cached connections after a period of inactivity.

We keep connections open to avoid re-doing the authentication procedure
when the user runs or deploys a remote app repeatedly.
However, the connections should not be around indefinitely; we therefore
now close them after about five minutes of inactivity.

Change-Id: Ie677eb133b618d34c08528da2f2bc221472316b3
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
CppEditor: First un-register, then register in CppModelManager

...on a file path change.

Change-Id: Id4f67c9f133e53ddb3fea6c0ec3322f87914469b
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>