Add override for client_mode option per connection

This allows same server to be client for one connection and server for another. Many use cases one one that we are using is for cross region secure traffic.

[a_region_db]
accept = 8003
connect = a_region_db.example.com:3306
client = no
[b_region_db]
accept = 8004
connect = b_region_tunnel.example.com:8004
client = yes