Move from bounds on low-level objects to BP Send bounds

[TheBlueMatt]

Rather than bounding the traits used in `OutputSweeper`, move the
bounds to where we actually require them - a top-level `Send` bound
on the `Deref`-to-`OutputSweeper` type itself.

Based on #3734

[ldk-reviews-bot]

I've assigned @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[joostjager]

Starting to understand how it works now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.52%. Comparing base (f015545) to head (61cd37d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3776   +/-   ##
=======================================
  Coverage   89.52%   89.52%           
=======================================
  Files         157      157           
  Lines      125450   125450           
  Branches   125450   125450           
=======================================
  Hits       112305   112305           
+ Misses      10445    10442    -3     
- Partials     2700     2703    +3     

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[TheBlueMatt]

Rebased after dependent PR got merged.