[MRG] feat: log when DUL idle timer expires

[r1b]

Reference issue

This makes troubleshooting timeouts a bit easier.

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
• Apps updated and tested (if relevant)

[codecov]

Codecov Report

Merging #603 (c00125e) into master (b8d6b94) will not change coverage.
The diff coverage is 100.00%.

❗ Current head c00125e differs from pull request most recent head 46a6c99. Consider uploading reports for the commit 46a6c99 to get more accurate results

@@            Coverage Diff            @@
##            master      #603   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         8411      8412    +1     
=========================================
+ Hits          8411      8412    +1     

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