Code functionality to exit gently on signal handlers.

[prasanna-ds]

Overview of the PR:

• Added signal handlers to the asyncio event loop.
• Update config on receiving the signal, stop the pyrogram client, stop the event loop and eventually close the event loop.
• Added an integration test to receive a signal from a thread to see that event loops have been stopped successfully.

[codecov]

Codecov Report

Merging #98 (277797b) into master (b42861f) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   98.72%   98.83%   +0.10%     
==========================================
  Files           5        5              
  Lines         157      171      +14     
==========================================
+ Hits          155      169      +14     
  Misses          2        2              

Impacted Files	Coverage Δ
media_downloader.py	98.41% <100.00%> (+0.19%)	⬆️

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[stale]

This issue has been automatically marked as stale because it has not had recent activity in the past 60 days. It will be closed if no further activity occurs in the next 7 days. Thank you for your contributions.