Refactor _process_results to support new DB engines

[dantownsend]

Resolves #1053

[dantownsend]

There's a small risk changing this.

In the past, the following query:

>>> await Band.select(Band.name, Band.manager.name)
[{'name': 'Pythonistas', 'manager.name': 'Guido'},
 {'name': 'Rustaceans', 'manager.name': 'Graydon'},
 {'name': 'C-Sharps', 'manager.name': 'Anders'}]

Executed this SQL:

>>> print(Band.select(Band.name, Band.manager.name))
"""
SELECT ALL
    "band"."name" AS "name", 
    "band$manager"."name" AS "manager$name"    <- NOTE THIS
FROM "band"
    LEFT JOIN "manager" "band$manager"
    ON ("band"."manager" = "band$manager"."id")
"""

Note how we have$ in the aliases - AS "manager$name".

$ was chosen because it's incredibly unlikely that someone would use $ in their column names, and it still works even without the quotes around it:

SELECT name AS foo$bar from band;
-- Returns [{"foo$bar": "Pythonistas"}, ...]

We can use . instead, as long as we're careful to wrap all of our aliases in quotes (otherwise the query fails).

SELECT name AS foo.bar from band;
-- Error!

So our query now becomes:

SELECT ALL
    "band"."name" AS "name", 
    "band$manager"."name" AS "manager.name"    -- <- NOTE THIS
FROM "band"
    LEFT JOIN "manager" "band$manager"
    ON ("band"."manager" = "band$manager"."id")

This means we don't have to convert from manager$name to manager.name in Piccolo, which saves some overhead.

I checked the codebase, and it looks like we wrap all aliases in double quotes. If there's an edge case where we don't, it's something we'll have to fix.