Add configuration for database connection pool validation

[guidograzioli]

It is now possible to set the behaviour for database connection validation using the following parameters:

Variable	Description	Default
keycloak_db_background_validation	Enable background validation of database connection	False
keycloak_db_background_validation_millis	How frequenly the connection pool is validated in the background	10000 if background validation enabled
keycloak_db_background_validate_on_match	Enable validate on match for database connections	False

Note that the keycloak setting configured with keycloak_db_background_validation is deprecated; also, having keycloak_db_background_validation set to false, and a non-zero keycloak_db_background_validation_millis will produce an incorrect configuration.

The default sql query used to validate the connection can be overridden defining the variable:

Variable	Description
keycloak_db_valid_conn_sql	Override the default database connection validation query sql

Fix #82