Add iterators for non pdo drivers.

statements in a rewindable iterator. Skipped the test for iterating
multiple times with a message regarding that iterators are non
rewindable.

Only added to mysqli for now, can add to other drivers (assuming they
support reseting the pointer in some way) after feedback.

Test fails because it's not possible to reuse a generator in this way.

I don't really understand the mysqli driver too much and the changes
break the tests so undo using an iterator for this particular driver.

Allows result sets to be iterated over without running out of memory on
large data sets.

Style improvements

use newer PHP syntax

use newer PHP syntax

Stop relying on Type::__toString

It is a somewhat fragile thing to do.

It groups things naturally.

Test default value declaration for the date type

It was not covered, probably because it was hard to do something
generic. I choose to duplicate some tests instead, so that things stay
relatively simple.

Update build to use stages (adding PHPCS to check for CS violations)

Modifying Travis-CI configuration to use  build stages and removing
explicit MySQL 5.6 execution (it's default now).

PHPCS is running with PHP nightly version so we can have it failing
for now.

This is now mandatory on the new Travis-CI containers.

use intended semantics

I think distinguish was the intended meaning here, not dismantle.

#2821 replacing `@group` annotations with proper references, since DBAL-* issues are from JIRA, not Github

Close #2821

Added upgrade notice about BC break.
Cleanups

Changed DBALException::invalidPlatformSpecified() exception to be more detailed about what was given.