Accept PDO as a Connection constructor argument

[morozov]

Q	A
Type	improvement
BC Break	no

This patch is a step towards sunsetting Connection::getWrappedConnection(). This method is not part of the driver API, so wrapping a driver connection into a decorator will make this method inaccessible. This is likely already a problem when the Portability middleware is used (#4157) and will become more obvious when the Logging middleware has been introduced (#4941).

Instead of instantiating PDO, the driver connection constructor will now accept it as a dependency. It has the following advantages:

1. It becomes possible to implement a driver that uses a custom PDO instance and reuse the rest of the PDO driver components (example). This solves the problem of sharing an underlying connection between multiple libraries w/o using a dedicated type-unsafe API.
2. The pdo_pgsql and pdo_sqlite drivers will no longer break their connection encapsulation by calling Connection::getWrappedConnection() in order to access the underlying PDO instance.

The general idea here is that every driver-level component should accept and wrap its lower-level counterpart, not instantiate it. This will allow for a more clear separation of concerns and, eventually, a cleaner internal API.

A similar change was applied recently in #4894.