Reworked AbstractPlatform::get*Expression() methods

[morozov]

Q	A
Type	improvement
BC Break	yes

This pull request contains the breaking changes in AbstractPlatform::get*Expression() methods from #3348.

The key changes are:

1. The arguments of the changed methods which previously could accept an SQL expression but weren't annotated so are now annotated.
2. The arguments which were declared as int but de-facto accepted an SQL expression are now type-hinted as string.
3. As a side effect of the above, with a slight rework in the implementation, the $interval argument of the getDate(Add|Sub)*Expression() methods can now accept an SQL expression which makes it possible to define the interval as a numeric literal (former behavior), prepared statement parameter, column name or technically any valid SQL expression 🔥
4. The natively unsupported date intervals (e.g. weeks in SQLite) are now converted in SQL instead of the client side. Hence some changes in assertions against generated SQL on some platforms.
5. DataAccessTest::testDateArithmetics() has been reworked into a series of independent tests each of which represents the interval in the query in three modes: numeric literal, prepared statement parameter and an SQL expression.
6. As a side effect of the rework of SqlitePlatform::getDateArithmeticIntervalExpression(), it no longer truncates the time part of the result of addition or subtraction of days, weeks, months, quarters or years. This issue can be filed and fixed additionally in master since it impacts the code portability.
7. The names of the arguments were renamed closer to their SQL analogs.
8. The behavior implemented in Allow dynamic intervals in DATE_ADD & DATE_SUB for SQLite #3019 is now by design.

[Ocramius]

LGTM (besides nits)