Accept a connection URI in `create_pool()` and `create_async_pool()`, with pool options readable from the URI query string.

create_pool() and create_async_pool() must accept a connection URI as first
positional argument, as connect() and asyncConnect() already do. Pool
options may also be given in the URI query string:

    mariadb.create_pool("mariadb://root:secret@localhost/test?min_size=5&max_size=20")