[5.x] 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.

I also fixed memcache test issues.

[codecov-commenter]

Codecov Report

Merging #1120 (e04847f) into 5.x (a8e8e97) will decrease coverage by 0.2%. The diff coverage is 80.8%.

@@           Coverage Diff           @@
##             5.x   #1120     +/-   ##
=======================================
- Coverage   94.8%   94.6%   -0.1%     
=======================================
  Files        296     314     +18     
  Lines      27370   29128   +1758     
=======================================
+ Hits       25929   27539   +1610     
- Misses      1441    1589    +148     

Impacted Files	Coverage Δ
guillotina/tests/fixtures.py	91.4% <60.0%> (ø)
guillotina/db/storages/pg.py	85.8% <81.9%> (-3.4%)	:arrow_down:
guillotina/tests/test_catalog.py	100.0% <100.0%> (ø)
guillotina/tests/test_postgres.py	100.0% <100.0%> (ø)
guillotina/tests/utils.py	90.4% <100.0%> (ø)
guillotina/db/storages/cockroach.py	74.5% <0.0%> (-1.5%)	:arrow_down:
guillotina/tests/__init__.py	100.0% <0.0%> (ø)
guillotina/__init__.py	100.0% <0.0%> (ø)
guillotina/tests/conftest.py	100.0% <0.0%> (ø)
... and 16 more