[DBAL-563] Add interface for sequence emulated identity platforms

Currently identity columns on Oracle are implemented with a [trigger and sequence workaround](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Platforms/OraclePlatform.php#L431) to be compatible with IDENTITY generator strategy in ORM. However, using this strategy, the last insert ID is never returned when persisting, as the sequence name generated by DBAL [is not passed to the ID-Generator instance in ORM](https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L450) and thus [not passed to the driver](https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Id/IdentityGenerator.php#L55) when calling `IdentityGenerator::generate()`. Therefore the Oracle driver always returns null in this case. This makes this strategy unusable.
A similar case is given in PostgreSQL where `SERIAL` identity columns need a sequence in order to work. There is a [hackish implementation](https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L453) available in ORM for this case which makes it work.
It is possible that other vendors (when implemented) encounter the same issue. For this reason I tried to create a general solution with this PR.
ORM needs to know which platforms do not have native support for identity columns but can emulate them with sequences. To prepare this I added an interface that identifies a platform being able to do so. Platforms implementing this interface have to return the name of the sequence used for an identity column in a table. ORM can later make use of this and pass it to the `IdentityGenerator` so that the underlying driver in return is able to supply the last insert ID.
Current implementing platforms are PostgreSQL and Oracle.