Emit SCAN event on finish

[TheTechromancer]

This PR adds some fields to the SCAN event:

• started_at
• finished_at
• status
• duration
• duration_seconds

Also, an additional SCAN event is now emitted at the end of the scan. This acts as proof that the scan finished, and will be useful in the up-and-coming BBOT server.

[codecov]

Codecov Report

Attention: Patch coverage is 96.03175% with 5 lines in your changes missing coverage. Please review.

Project coverage is 93%. Comparing base (4a06dc9) to head (c58cf71).
Report is 41 commits behind head on dev.

Files with missing lines	Patch %	Lines
bbot/scanner/scanner.py	92%	5 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             dev   #1761    +/-   ##
======================================
- Coverage     93%     93%    -0%     
======================================
  Files        344     346     +2     
  Lines      26507   26712   +205     
======================================
+ Hits       24440   24625   +185     
- Misses      2067    2087    +20     

Flag	Coverage Δ
	93% <97%> (?)

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