[master] No longer have dedicated read connection and do not use any prepared statements

[vangheem]

The purpose of this PR is to get rid of our dedicated read connection and use of prepared statements:

• use of dedicated read conn for subset of queries can in some cases cause some lock contention between transactions
• use of prepared statements prevents people from using pgbouncer in transaction mode. asyncpg already automatically creates prepared statements for all sql. By removing the use of prepared statements, you can now set the statement cache size to 0 for asyncpg and have guillotina work with pgbouncer.

[vangheem]

hmmm, some 3.8 tests failing. Something unrelated is wrong in tests it seems to me.

[codecov-commenter]

Codecov Report

Merging #1121 (b477135) into master (c2eba32) will decrease coverage by 5.2%. The diff coverage is 16.7%.

:exclamation: Current head b477135 differs from pull request most recent head 4e2ec1e. Consider uploading reports for the commit 4e2ec1e to get more accurate results

@@           Coverage Diff            @@
##           master   #1121     +/-   ##
========================================
- Coverage    94.4%   89.3%   -5.1%     
========================================
  Files         372     372             
  Lines       31919   31333    -586     
========================================
- Hits        30131   27969   -2162     
- Misses       1788    3364   +1576     

Impacted Files	Coverage Δ
guillotina/tests/test_catalog.py	60.6% <0.0%> (-39.4%)	:arrow_down:
guillotina/tests/test_postgres.py	10.5% <20.0%> (-89.5%)	:arrow_down:
guillotina/db/storages/pg.py	34.9% <21.8%> (-53.6%)	:arrow_down:
guillotina/tests/test_cockroach.py	17.7% <0.0%> (-77.9%)	:arrow_down:
guillotina/api/aggregation.py	24.2% <0.0%> (-69.1%)	:arrow_down:
guillotina/db/storages/vacuum.py	47.0% <0.0%> (-53.0%)	:arrow_down:
guillotina/db/storages/cockroach.py	28.5% <0.0%> (-52.0%)	:arrow_down:
guillotina/contrib/catalog/pg/utility.py	24.9% <0.0%> (-49.7%)	:arrow_down:
guillotina/db/factory.py	47.0% <0.0%> (-45.9%)	:arrow_down:
... and 152 more