Max connections & max queue connections per backend server

[splushii]

Adds the ability to specify max number of connections and max number of queued connections per backend server in an ORM rule.

Example:

backend:
  max_connections:
  servers:
    - server: server1.example.com
      max_connections: 100
      max_queued_connections: 1000
    - server: server2.example.com
      max_connections: 123
      max_queued_connections: 10000

Implemented using HAProxy:s maxconn and maxqueue.