DB improvements

[boekkooi-fresh]

This PR improves:

• the projector query to have a lower cost
• To only use a transaction when inserting events when need

[codecov]

Codecov Report

Merging #34 into master will decrease coverage by 0.01%.
The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   45.12%   45.11%   -0.02%     
==========================================
  Files          37       37              
  Lines        2245     2250       +5     
==========================================
+ Hits         1013     1015       +2     
- Misses       1201     1203       +2     
- Partials       31       32       +1

Impacted Files	Coverage Δ
driver/sql/postgres/eventstore.go	78.91% <0%> (-1.46%)	⬇️
driver/sql/postgres/projector_aggregate_storage.go	42.17% <100%> (+0.79%)	⬆️

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