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.

Fixed cs issues

fixing issue #2814

Replaced the code examples for `set()` to show that the value parameter is not escaped and thus could pose an SQL injection vulnerability.

fixed return type into OCI8Statement

Fix `CompositeExpression#add()` optimization, which was leading to bbroken `CompositeExpression` instances

Fix inconsistent/missing return statements

Use short array declarations

Typing final class DriverManager

add @throws declarations to docs in Connection

Fix PSR2 rules : "the static declaration should come after visibility"

Fix Shard Manager docs with more accurate docblocks in `SQLAzureShardManager` and `PoolingShardManager`

In a few cases methods in Connection may throw a DBALException which
isn't declared in the docblock. As the Connection class is heavily
used by third party code, this code runs the risk of ignoring
those exceptions instead of handling them. While it is
technically not required to declare exceptions in PHP, it is highly
recommended, especially in public APIs.

[TASK] Fix articles of words sounding with a consonant

Fix PSR2 rules : "As per PSR2, the static declaration should come after the visibility declaration."

Close #2794

#2794 allowing `DateTimeImmutable` instances when converting in mutable date types - reverts unintentional BC break