[MRG] Allow A-RELEASE instead of A-ABORT on network timeout

[scaramallion]

Reference issue

• Closes Release instead of Abort when network_timeout expires #619
• Add Association.network_timeout_response to allow A-RELEASE instead of A-BORT

Tasks

• Unit tests added that reproduce issue or prove feature is working
• Fix or feature added
• Documentation and examples updated (if relevant)
• Unit tests passing and coverage at 100% after adding fix/feature
• Type annotations updated and passing with mypy

[codecov]

Codecov Report

Merging #725 (ddeb640) into master (7a0c87b) will not change coverage.
The diff coverage is 100.00%.

❗ Current head ddeb640 differs from pull request most recent head dab1766. Consider uploading reports for the commit dab1766 to get more accurate results

@@            Coverage Diff            @@
##            master      #725   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines         8642      8647    +5     
=========================================
+ Hits          8642      8647    +5     

Impacted Files	Coverage Δ
pynetdicom/dul.py	100.00% <ø> (ø)
pynetdicom/association.py	100.00% <100.00%> (ø)