DB agnostic `ForeignKeyConstraint::getOnUpdate()` and `ForeignKeyConstraint::getOnDelete()`

[arogachev]

Example for NO ACTION:

• SQLite, latest MySQL and PostgreSQL - NO ACTION.
• MySQL 5.7 - RESTRICT.
• MS SQL - NOACTION.
• Oracle - null / NO ACTION.

[Tigrov]

Looks like predefined constants with appropriate values for each DBMS are needed. Related with #659?