fix: race condition with Sequelize, feat: support multiple SET statements in one query

[levkk]

Race condition fix

Fixes race condition with sequelize and other similar ORMs that do the following:

Parse
Describe
Flush
Bind
Execute
Flush

We treated this as query is finished and let our pool logic send Sync asynchronously as a cleanup step. Meanwhile, Sequelize would send Sync separately and we would immediately return ReadyForQuery, but the cleanup is async and would happen after we responded with ReadyForQuery. The client then thought a record was created before it was actually saved in Postgres. This is a likely fix for #685 cc @mkreidenweis-schulmngr

Multiple SET statements

Sequelize also sends multiple SET commands in one query. We handle this now correctly, although it's not related to the race condition.

[codecov]

Codecov Report

❌ Patch coverage is 95.77465% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/query/set.rs	92.75%	5 Missing ⚠️
.../src/frontend/router/parser/query/test/test_set.rs	93.44%	4 Missing ⚠️
.../frontend/router/parser/query/test/test_special.rs	88.88%	3 Missing ⚠️

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