Update min Swift version to 5.6 and make platform versions consistent

This patch was authored and released by @gwynne.

Update to match FluentKit's declared version minimums

This patch was authored and released by @gwynne.

This should help fix building in Xcode.

Update minimum Swift version to 5.5

This patch was authored and released by @gwynne.

And a couple of minor cleanups to tests.

Add ability to control transactions

This patch was authored and released by @0xTim.

Adds the ability to control starting, committing and rolling back transactions outside of the main Fluent API. This could be used for setting up tests.

Since the new APIs give you manual control over a transaction, you should not use Database.transaction(_:) when using the manual transaction control. The inTransactionFlag will not be set when manually controlling a transaction

Warning: It is the users' responsibility to ensure the handle errors and rollback when necessary and commit transactions

Related to vapor/fluent-kit#520

Explicitly handle a custom ID key set to the empty string

This patch was authored and released by @gwynne.

Treat it as meaning not to retrieve an inserted ID value. This is in support of upcoming FluentKit feature work.

Clean up row decoding

This patch was authored and released by @gwynne.

Instead of duplicating logic implemented by postgres-kit for row decoding - and incorrectly, at that - we now always call through to that logic. Despite the additional indirection through the SQLRow existential, this should yield a minor improvement in performance, as we perform fewer unnecessary checks and retain fewer copies of data.

Note: Depends on vapor/postgres-kit#221 to receive the full benefit of these changes.

Fix logging of enum builder queries

This patch was authored and released by @gwynne.

Queries issued for creation, updating, and dropping of enum types in databases which use custom data types for enumeration support (e.g. Postgres) were not being logged properly.

Support adding multiple enum cases at once

This patch was authored by @seeppp and released by @0xTim.

Fixes an issue where updating a table to add multiple new enum cases would only add the last case to the database.

Fixes #189

Bump postgres-kit to 2.5.1

This patch was authored by @sketchmaster2001 and released by @0xTim.

postgres-kit has been updated to fix build conflicts with swift-nio. This bumps the postgres-kit version to fix build errors.

Make `DatabaseError.isConnectionClosed` functional

This patch was authored and released by @gwynne.

DatabaseError.isConnectionClosed now has the value true when the underlying error is PostgresError.connectionClosed.

Also avoids double-running the FluentBenchmark tests (same change as fluent-sqlite-driver#75) and actually runs the tests on Linux now.