Use AChannelManager in BackgroundProcessor

[jkczyz]

Replace instance of ChannelManager in BackgroundProcessor and inPersister with AChannelManager. This reduces the number of type parameters need in those types, which would need to be repeated in an async version of Persister.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.21%. Comparing base (670b41a) to head (d29e2ba).
Report is 68 commits behind head on main.

Files	Patch %	Lines
lightning/src/util/persist.rs	50.00%	2 Missing ⚠️
lightning-background-processor/src/lib.rs	92.85%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2963      +/-   ##
==========================================
+ Coverage   89.11%   90.21%   +1.10%     
==========================================
  Files         117      117              
  Lines       95029   101020    +5991     
  Branches    95029   101020    +5991     
==========================================
+ Hits        84685    91140    +6455     
+ Misses       7856     7597     -259     
+ Partials     2488     2283     -205     

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[tnull]

LGTM, tested LDK Node tests pass based on this.