[marshal] allow for sync-height floor

[BrendanChou]

Fixes #1706

TODO: tests

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.09%. Comparing base (7a66af6) to head (45c6a23).

Files with missing lines	Patch %	Lines
consensus/src/marshal/finalizer.rs	72.72%	3 Missing ⚠️
consensus/src/marshal/actor.rs	85.71%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main    #1707      +/-   ##
==========================================
- Coverage   92.10%   92.09%   -0.01%     
==========================================
  Files         297      297              
  Lines       75500    75516      +16     
==========================================
+ Hits        69538    69550      +12     
- Misses       5962     5966       +4     

Files with missing lines	Coverage Δ
consensus/src/marshal/mod.rs	99.75% <100.00%> (ø)
consensus/src/marshal/actor.rs	87.04% <85.71%> (-0.06%)	⬇️
consensus/src/marshal/finalizer.rs	85.18% <72.72%> (-3.46%)	⬇️

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[patrick-ogrady]

We need to be sure we cancel all resolution requests if we allow the sync target to update dynamically (or track height + digest).