Support async signing for V2 channel establishment

[jkczyz]

When handling a tx_complete message, allow signers to return an error indicating that the signer has not yet complete. This will leave the ChannelPhase in an unfunded variant until the signer becomes unblocked. The user calls ChannelManager::signer_unblocked to indicate that signing is complete, which will attempt to finish handling the tx_complete message again.

Based on #3137. Fixes #3404.

[jkczyz]

Fixed a couple rustfmt issues in dual_funding_tests.rs. Also, removed a ton of unnecessary drains from channelmanager.rs for good measure since I was looking at related code and noticed a Vec wasn't being used, which I removed in another commit, too.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.91176% with 45 lines in your changes missing coverage. Please review.

Project coverage is 89.23%. Comparing base (0c31021) to head (6db89d7).

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	64.61%	16 Missing and 7 partials :warning:
lightning/src/ln/channel.rs	62.50%	20 Missing and 1 partial :warning:
lightning/src/ln/interactivetxs.rs	50.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3411      +/-   ##
==========================================
+ Coverage   89.22%   89.23%   +0.01%     
==========================================
  Files         130      130              
  Lines      106965   106958       -7     
  Branches   106965   106958       -7     
==========================================
+ Hits        95438    95446       +8     
+ Misses       8734     8725       -9     
+ Partials     2793     2787       -6     

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[TheBlueMatt]

Tagging 0.2 since we want to include both full async signing and v2 channel establishment in it.

[TheBlueMatt]

We ultimately decided to push both dual funding and async signing to 0.3.