* Replace connection handle method check_connection() with is_connected() and ping_server()
* When a connections is fetched from a pool perform a validity check which can be one of none, passive or ping.
* Add the methods is_connected() and ping_server() to the connection template class.
* Remove unused #include and add mising #include.
* Add tests for the connection methods is_connected() and ping_server().
* Add sqlpp::compat::make_unique which calls std::make_unique in C++14 or newer and falls back to a custom implementation in C++11 mode.
* Add tests for sqlpp::compat::make_unique
Add support for connection pooling
* Add support for connection pooling to the core code.
* Add support for PostgreSQL connection pooling with tests.
* Add support for SQLite3 connection pooling with tests.
* Add support for MySQL connection pooling with tests.
