Modularised and added Oracle and SQL Server backends

[milessabin]

• Split sql module into sql-core and sql-pg, the latter containing Postgres specfics shared by doobie-pg and skunk.
• Split off doobie-core from doobie-pg removing all Postgres specfics from the former.
• Factored out all Postgres specifics from SqlMapping.
• Added Oracle backend.
• Added SQL Server backend.
• Reduced the use of lateral subqueries to a minimum.
• Simplified SqlSelect nesting logic.
• Avoid nested conditions being captured by predicate subqueries.
• SkunkMapping now instantiated with a Session rather than a session Resource to avoid resource lifecycle on every query. update: this turned out to be a Bad Idea and has been reverted.
• Replaced whale-tail with call outs to docker-compose allowing container to be shared by all suites across multiple test runs.

Running tests (eg. rootJVM/test) will automatically spin up the relevant containers, which will stay up and can be reused across multiple tests runs, significantly speeding up the test cycle. I recommend running allUp initially, to pull images and initialise databases, before running tests the first time. Note that Oracle in particular takes quite a long time to initialise, so expect allUp to take several minutes to complete the first time around.

There have been some changes in compilation of queries for Postgres, primarily a reduction in the usage of lateral subqueries (partly to simplify the implementation for SQL Server, which has a subtly different mechanism for achieving similar effects). These changes should preserve expected results, and reports of any changes in behaviour, including performance regressions, would be very much appreciated.

[phdoerfler]

This is great news!

While checking out this PR, I noticed the following:
The docker-compose file uses start_interval in the health check section of oracle and mssql. This requires at least Docker 2.20.2. I think it is worth documenting this in the readme or else one may be greeted by services.mssql.healthcheck Additional property start_interval is not allowed when trying to run the tests.
On that note: Even though this failure prevents docker from starting, the tests are executed regardless resulting in many test failures. In addition, the ammount of output on the console makes the error message hard to miss. It probably would not hurt to check for status code 0 when launching docker compose to escalate any problem to sbt.

[milessabin]

Agreed ... I'll add a note in the README.

[milessabin]

@tpolecat Skunk changes made ...

@phdoerfler docs updated ...

[milessabin]

👍's from @tpolecat, @CamiloMorales and @phdoerfler ...