persister: Expose method to read ChannelMonitors from disk

[valentinewallace]

Tested w/ the sample.

[codecov]

Codecov Report

Merging #863 (94aa47b) into main (df732f4) will increase coverage by 1.89%.
The diff coverage is 87.09%.

@@            Coverage Diff             @@
##             main     #863      +/-   ##
==========================================
+ Coverage   90.59%   92.49%   +1.89%     
==========================================
  Files          51       51              
  Lines       27195    32604    +5409     
==========================================
+ Hits        24638    30157    +5519     
+ Misses       2557     2447     -110     

Impacted Files	Coverage Δ
lightning-persister/src/lib.rs	94.95% <87.09%> (-0.66%)	⬇️
background-processor/src/lib.rs	98.73% <0.00%> (-1.27%)	⬇️
lightning/src/routing/router.rs	96.87% <0.00%> (-0.10%)	⬇️
lightning/src/ln/reorg_tests.rs	99.63% <0.00%> (+0.06%)	⬆️
lightning/src/util/test_utils.rs	83.23% <0.00%> (+0.28%)	⬆️
lightning/src/ln/functional_test_utils.rs	96.07% <0.00%> (+0.85%)	⬆️
lightning/src/ln/functional_tests.rs	98.47% <0.00%> (+1.58%)	⬆️
lightning/src/ln/channel.rs	91.87% <0.00%> (+4.74%)	⬆️
lightning/src/ln/channelmanager.rs	89.25% <0.00%> (+6.02%)	⬆️

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[jkczyz]

Should this be a NotFound error? That way caller's can create the directory if it does not exist.

[TheBlueMatt]

Hmm, isn't it more likely its just the first startup and thus there's no monitors?

[jkczyz]

I was thinking that the caller would handle creating the directory. But I guess that FilesystemPersister creates it on demand, so probably ok then. Wasn't sure if we want to give the user insight into this (e.g., they instantiate a persister expecting to find data but don't because the path was wrong).