MariaDB: add dthread support

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: fix empty result set issue

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: add documentation

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: update APIs

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: update dthread doc

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: update async api doc

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: add pool support to dthread db connection

Signed-off-by: swpd <swpdtz@gmail.com>
MariaDB: add documentation for dthread_pool_get_conn

Signed-off-by: swpd <swpdtz@gmail.com>
Merge pull request #22 from swpd/mariadb_dthread

Mariadb Dthread Support