Fix outbound payments probe leak

[valentinewallace]

Fixes #2036.

Based on #2008.

[codecov-commenter]

Codecov Report

Base: 87.21% // Head: 87.02% // Decreases project coverage by -0.20% ⚠️

Coverage data is based on head (2f14891) compared to base (30b9d9f).
Patch coverage: 92.70% of modified lines in pull request are covered.

❗ Current head 2f14891 differs from pull request most recent head a2b956d. Consider uploading reports for the commit a2b956d to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2037      +/-   ##
==========================================
- Coverage   87.21%   87.02%   -0.20%     
==========================================
  Files         100      100              
  Lines       44023    44543     +520     
  Branches    44023    44543     +520     
==========================================
+ Hits        38396    38762     +366     
- Misses       5627     5781     +154     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	85.84% <ø> (ø)
lightning/src/routing/router.rs	92.79% <ø> (ø)
lightning/src/util/events.rs	23.63% <ø> (-0.58%)	⬇️
lightning/src/ln/payment_tests.rs	95.94% <82.05%> (+<0.01%)	⬆️
lightning/src/ln/outbound_payment.rs	80.40% <84.21%> (+0.11%)	⬆️
lightning/src/ln/chanmon_update_fail_tests.rs	97.31% <100.00%> (ø)
lightning/src/ln/functional_test_utils.rs	88.50% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	97.29% <100.00%> (+0.01%)	⬆️
lightning/src/ln/monitor_tests.rs	98.73% <100.00%> (ø)
lightning/src/ln/onion_route_tests.rs	95.45% <100.00%> (ø)
... and 11 more

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[TheBlueMatt]

Needs rebase now that #2008 landed.

[TheBlueMatt]

Change LGTM, though.