Add connection error simulation support

[masaori335]

It'll be very useful to test origin server side connection error handling of the proxy .

An idea of schema is adding new connection directive accepts the following values:

Value	Description
accept	Normal behavior - accept the connection and process the transaction (default).
refuse	Simulate connection refused - close the connection immediately after reading the request without sending any response.
reset	Simulate connection reset - send a TCP RST after reading the request.

• e.g.

sessions:
- transactions:
  - client-request:
      method: GET
      url: /path/

    server-response:
      connection: refuse  

    proxy-response:
      status: 503