Simplify CertStore subscriber

[Stebalien]

I have yet to write any code that cares about intermediate certificates and they can easily be queried with GetRange. This change:

1. Makes the certificate subscription system always return the latest certificate, dropping intermediates.
2. Drops the dependency on go-broadcast.
3. Simplifies the code that relies on the certificate subscription logic.

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.67%. Comparing base (92a5658) to head (46ff473).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
certstore/certstore.go	96.77%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #646      +/-   ##
==========================================
- Coverage   80.02%   79.67%   -0.35%     
==========================================
  Files          52       52              
  Lines        4695     4703       +8     
==========================================
- Hits         3757     3747      -10     
- Misses        590      603      +13     
- Partials      348      353       +5     

Files with missing lines	Coverage Δ
host.go	69.47% <100.00%> (-1.56%)	⬇️
certstore/certstore.go	68.44% <96.77%> (+2.31%)	⬆️

... and 5 files with indirect coverage changes