Change ChannelManager deserialization to return an optional blockhash

[valentinewallace]

If the ChannelManager never receives any blocks, it'll return a default blockhash
on deserialization. It's preferable for this to be an Option instead.

[codecov]

Codecov Report

Merging #810 (c21b616) into main (59d2040) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #810   +/-   ##
=======================================
  Coverage   91.02%   91.02%           
=======================================
  Files          48       48           
  Lines       25495    25501    +6     
=======================================
+ Hits        23207    23213    +6     
  Misses       2288     2288           

Impacted Files	Coverage Δ
lightning-block-sync/src/init.rs	93.56% <ø> (ø)
lightning-persister/src/lib.rs	92.52% <100.00%> (ø)
lightning/src/chain/channelmonitor.rs	95.77% <100.00%> (+<0.01%)	⬆️
lightning/src/ln/chanmon_update_fail_tests.rs	97.56% <100.00%> (ø)
lightning/src/ln/channelmanager.rs	85.28% <100.00%> (+0.02%)	⬆️
lightning/src/ln/functional_test_utils.rs	94.97% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	96.96% <100.00%> (ø)
lightning/src/util/test_utils.rs	83.49% <100.00%> (ø)

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[TheBlueMatt]

Should probably be done in ChannelMonitors too, no?

[TheBlueMatt]

Needs rebase and the check_commits CI failed.

[TheBlueMatt]

There is no fuzz issue that I can see (are you running cargo test in fuzz with the newly-required RUSTFLAGS="--cfg=fuzzing"?). I pushed a rebased version of this at https://git.bitcoin.ninja/?p=rust-lightning;a=shortlog;h=refs/heads/2021-03-810-rebased.

I also included a commit which updates the docs there, and I'm ACK on the version available there.

[TheBlueMatt]

See #819.