Cache timestamp conversion

[bdraco]

We end up converting the same data over and over since we get 72 new activities every time we poll the api, and we likely have seen 71 of them before

• removes some duplicate code and updates some more typing

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (9572fe1) 95.55% compared to head (319f5aa) 95.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   95.55%   95.56%   +0.01%     
==========================================
  Files          29       29              
  Lines        3305     3315      +10     
==========================================
+ Hits         3158     3168      +10     
  Misses        147      147              

Flag	Coverage Δ
unittests	95.56% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
yalexs/util.py	96.00% <ø> (ø)
yalexs/activity.py	96.56% <100.00%> (-0.03%)	⬇️
yalexs/api.py	90.83% <100.00%> (+0.07%)	⬆️
yalexs/api_async.py	89.44% <100.00%> (+0.06%)	⬆️
yalexs/api_common.py	93.23% <100.00%> (+0.05%)	⬆️
yalexs/authenticator.py	73.43% <100.00%> (+0.42%)	⬆️
yalexs/authenticator_async.py	72.72% <100.00%> (ø)
yalexs/authenticator_common.py	83.87% <100.00%> (+0.17%)	⬆️
yalexs/exceptions.py	100.00% <100.00%> (ø)
yalexs/lock.py	94.65% <100.00%> (-0.03%)	⬇️
... and 4 more

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