WIP: Drop ChannelManager's ChannelMonitor Arc for reference

[valentinewallace]

Blocked by #423. Also probs should try to break into multiple commits.

Partially solves #237.

[codecov]

Codecov Report

Merging #424 into master will increase coverage by 0.17%.
The diff coverage is 99.49%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
+ Coverage   87.49%   87.66%   +0.17%     
==========================================
  Files          30       30              
  Lines       16733    16955     +222     
==========================================
+ Hits        14640    14864     +224     
+ Misses       2093     2091       -2

Impacted Files	Coverage Δ
lightning/src/util/test_utils.rs	53.33% <ø> (ø)	⬆️
lightning/src/ln/channelmanager.rs	80.56% <100%> (ø)	⬆️
lightning/src/ln/chanmon_update_fail_tests.rs	97% <100%> (+0.08%)	⬆️
lightning/src/ln/peer_handler.rs	32.83% <100%> (+1.22%)	⬆️
lightning/src/chain/chaininterface.rs	87.38% <100%> (-0.44%)	⬇️
lightning/src/ln/functional_tests.rs	95.83% <100%> (+0.22%)	⬆️
lightning/src/ln/channelmonitor.rs	85.03% <66.66%> (ø)	⬆️
lightning/src/ln/functional_test_utils.rs	94.7% <96.77%> (+0.06%)	⬆️

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