Offset and event de-duplication handling for Nostr events

[tompro]

Part of #119

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 15 lines in your changes missing coverage. Please review.

Project coverage is 39.83%. Comparing base (1c0fe9c) to head (ed75a7d).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/persistence/db/nostr_event_offset.rs	84.44%	7 Missing ⚠️
src/util/date.rs	63.63%	4 Missing ⚠️
src/service/notification_service/nostr.rs	93.61%	3 Missing ⚠️
src/persistence/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
+ Coverage   39.22%   39.83%   +0.60%     
==========================================
  Files          53       56       +3     
  Lines       10212    10305      +93     
==========================================
+ Hits         4006     4105      +99     
+ Misses       6206     6200       -6     

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