Persist ChannelManager before BackgroundProcessor exits

[TheBlueMatt]

Fixes #1237.

[codecov-commenter]

Codecov Report

Merging #1253 (ce82a33) into main (7b6a7bb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1253      +/-   ##
==========================================
+ Coverage   90.41%   90.42%   +0.01%     
==========================================
  Files          70       70              
  Lines       38087    38087              
==========================================
+ Hits        34437    34441       +4     
+ Misses       3650     3646       -4     

Impacted Files	Coverage Δ
lightning-background-processor/src/lib.rs	92.98% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	97.36% <0.00%> (+0.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b6a7bb...ce82a33. Read the comment docs.