feat(store): add Tail method

[cristaloleg]

Overview

Add .Tail method to header.Store[H] and store.Store[H] implementation. For now method tries to return genesis header. In the future PRs method will start returning real tail of the store.

Updates #203

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 26.31579% with 14 lines in your changes missing coverage. Please review.

Please upload report for BASE (backward-sync@7d9aad6). Learn more about missing BASE report.

Files	Patch %	Lines
headertest/store.go	0.00%	8 Missing :warning:
store/store.go	45.45%	6 Missing :warning:

Additional details and impacted files

@@               Coverage Diff                @@
##             backward-sync     #210   +/-   ##
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  Coverage                 ?   63.32%           
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  Files                    ?       38           
  Lines                    ?     3610           
  Branches                 ?        0           
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  Hits                     ?     2286           
  Misses                   ?     1155           
  Partials                 ?      169           

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