feat(chain): add ChainPosition::blocks_since_conf

[Dmenec]

Suggested by @nymius in #2246 (comment).

Description

Adds ChainPosition::blocks_since_conf(tip), which returns the number of blocks mined on top of a confirmed position's block, given the chain tip. Returns None if the position is unconfirmed, or if the confirmation height is above tip.

Changelog notice

• Added ChainPosition::blocks_since_conf, returning the number of blocks mined on top of a confirmed position's block given the chain tip.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.41%. Comparing base (456f9b7) to head (defff8e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2263      +/-   ##
==========================================
+ Coverage   78.36%   78.41%   +0.04%     
==========================================
  Files          30       30              
  Lines        5945     5952       +7     
  Branches      281      282       +1     
==========================================
+ Hits         4659     4667       +8     
+ Misses       1210     1209       -1     
  Partials       76       76              

Flag	Coverage Δ
rust	78.41% <100.00%> (+0.04%)	⬆️

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