* 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 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.
Leak: The sqlite3 connection handle destructor was
accidentally turned into a default constructor during the
migration of repositories.
Replaced the sqlite3 raw pointer with a unique_ptr to prevent
this kind of accident.
Add Connector Cmake logic
* Also install date when used with fetch content
* Install everything always
* Update documentation
* Add option to control dependency searching
* Adjust travis
Changed sqlite3 usage tests to be linked into one executable.
This provokes multiple definition errors for non-inlined free
functions.
Also ran clang-format on the changed headers.
