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PDO refactoring

PDO extension is written long ago and did not evolve for years. There is a list of problems with it: * Absence of non-blocking queries compatible with lib/event * Violation of a Single Responsibility and Command-Query Separation principles in design * Weak placeholder implementation * Difficult query debugging with parameterized queries * Lack of plugins for Redis and modern databases Database operations is an essential part of all applications. It need attention.