Use connection pool when possible (#1122)

[masipcat]

Ported https://github.com/plone/guillotina/pull/1122

[codecov-commenter]

Codecov Report

Merging #1154 (5d71548) into master (86d0b29) will decrease coverage by 0.6%. The diff coverage is 90.0%.

@@           Coverage Diff            @@
##           master   #1154     +/-   ##
========================================
- Coverage    94.6%   94.0%   -0.5%     
========================================
  Files         383     377      -6     
  Lines       32775   32593    -182     
========================================
- Hits        30995   30633    -362     
- Misses       1780    1960    +180     

Impacted Files	Coverage Δ
guillotina/db/interfaces.py	100.0% <ø> (ø)
guillotina/db/storages/dummy.py	94.0% <ø> (-<0.1%)	:arrow_down:
guillotina/factory/app.py	94.2% <ø> (-<0.1%)	:arrow_down:
guillotina/tests/test_transactions.py	100.0% <ø> (ø)
guillotina/db/transaction.py	89.6% <80.0%> (-0.7%)	:arrow_down:
guillotina/db/storages/pg.py	84.3% <90.3%> (-1.6%)	:arrow_down:
guillotina/db/storages/base.py	97.8% <100.0%> (-<0.1%)	:arrow_down:
guillotina/db/storages/cockroach.py	28.9% <100.0%> (-50.9%)	:arrow_down:
guillotina/factory/content.py	83.3% <100.0%> (ø)
guillotina/tests/mocks.py	85.4% <100.0%> (-4.4%)	:arrow_down:
... and 13 more