Handle monotonic clock going backwards during runtime

[TheBlueMatt]

We've had some users complain that duration_since is panic'ing
for them. This is possible if the machine being run on is buggy and
the "monotonic clock" goes backwards, which sadly some ancient
systems can do.

Rust addressed this issue in 1.60 by forcing
Instant::duration_since to not panic if the machine is buggy
(and time goes backwards), but for users on older rust versions we
do the same by hand here.

[codecov-commenter]

Codecov Report

Merging #1692 (82474d3) into main (b84b53a) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1692      +/-   ##
==========================================
+ Coverage   90.77%   90.83%   +0.06%     
==========================================
  Files          80       85       +5     
  Lines       44763    46221    +1458     
  Branches    44763    46221    +1458     
==========================================
+ Hits        40632    41987    +1355     
- Misses       4131     4234     +103     

Impacted Files	Coverage Δ
lightning/src/util/time.rs	81.53% <100.00%> (+0.28%)	⬆️
lightning/src/chain/keysinterface.rs	92.54% <0.00%> (-1.06%)	⬇️
lightning/src/ln/wire.rs	61.99% <0.00%> (-0.93%)	⬇️
lightning/src/chain/onchaintx.rs	94.02% <0.00%> (-0.89%)	⬇️
lightning/src/util/config.rs	66.66% <0.00%> (-0.78%)	⬇️
lightning/src/util/test_utils.rs	77.49% <0.00%> (-0.62%)	⬇️
lightning/src/ln/onion_utils.rs	94.88% <0.00%> (-0.57%)	⬇️
lightning/src/ln/msgs.rs	85.74% <0.00%> (-0.54%)	⬇️
lightning/src/ln/peer_handler.rs	56.76% <0.00%> (-0.50%)	⬇️
lightning/src/ln/monitor_tests.rs	99.55% <0.00%> (-0.45%)	⬇️
... and 32 more

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