Abstract away pg

[gajus]

The primary goal of this refactor is to remove hard dependency on pg.

My intention is to move Slonik away from being a pg wrapper, to make it a universal client. The work in this PR abstracts all interactions with the database behind a pool and drivers (e.g. createPgDriver). This makes it very easy to implement drivers for pg-native, postgres and other drivers.

• Drops dependency on pg.Pool
  • Implements a more efficient internal pooling mechanism.
  • For context, test execution time dropped from ~50 seconds down to ~6 seconds.
• Renames getPoolState() to state()
• Removes createMockPool
  • It was at best not useful, and at worst misleading. Even Slonik itself didn't use it. There are better ways to mock/spy connections. Refer to createPoolWithSpy as one of the possible ways.
  • Refactors all tests to use database connection instead of mocking.

[github-actions]

🎉 This PR is included in version 39.0.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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