Cache mysql connections

Hash table uri -> Mysql_connection to have only one connection per mysql server used.

It requires to have samba-libs package with libccan.
@blaxter  I would suggest that we don't remove this, but replace it with something like release_connection()
release_connection() is going to end in mysql.c module as doing nothing at the moment, but this
should keep the parity on connection handling from mysql.c module clients -> create/release

What do you think?