Update manifest defaults for mainnet

[Stebalien]

• From testing, we need a 6s delta.
• This configures the alignment to be 15s which is what we used in testing.
• It adjusts the rebroadcast timings to align with the new delta.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.61%. Comparing base (2ab3d1d) to head (d973364).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   76.59%   76.61%   +0.01%     
==========================================
  Files          65       65              
  Lines        5401     5401              
==========================================
+ Hits         4137     4138       +1     
+ Misses        865      863       -2     
- Partials      399      400       +1     

Files with missing lines	Coverage Δ
manifest/manifest.go	58.77% <ø> (ø)

... and 2 files with indirect coverage changes