Add new scan states for scan sessions that are no longer accessible by client

[keith-turner]

Is your feature request related to a problem? Please describe.

Scan session that are no longer accessible by a client may still show in list scans. These scan session could be in the process of cleaning up resources or have a threads running. There is currently no direct indication of this in list scans output. There is an indirect indication that was not intentional , currently inaccessible scan session will lose their scan id. When this happens they will show up in listscans output with an id of zero or one. If #4842 were implemented this would no longer be the case.

Describe the solution you'd like

Update ScanState enum to include one or two new states. These new states could be CLEANING and ZOMBIE possibly. The cleaning state would indicate the session is no longer accessible and is in the process of cleaning up resources. The zombie state would indicate the client session is no longer accessible and still has a thread running on the server side.

After the changes in #4840, the information is available server side to populate new scan states.

[ArbaazKhan1]

I can take a look at this