fix(store): properly update store head

[cristaloleg]

Overview

The main idea of this PR is to make Store[H].Head working properly, to be precise: returning head that was written to the disk (*). Along with that heightSub.height is increased monotonically to prevent bugs when we have store appends out of order.

To test everything I'm adding 2 new tests: one that verifies out of order appends and another when which does this concurrently. Which helped to find 2 or even 3 edge cases during coding.

Fixes #201

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.54054% with 7 lines in your changes missing coverage. Please review.

Project coverage is 63.82%. Comparing base (88c5b8c) to head (e552672).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
store/store.go	93.10%	2 Missing and 2 partials ⚠️
store/heightsub.go	81.25%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
+ Coverage   62.80%   63.82%   +1.01%     
==========================================
  Files          39       38       -1     
  Lines        3589     3621      +32     
==========================================
+ Hits         2254     2311      +57     
+ Misses       1160     1141      -19     
+ Partials      175      169       -6     

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[walldiss]

Looks good 👍