journal-gatewayd: allow pipelining

journal-gatewayd: allow pipelining

The request must not be answered immediately (at first call to
response_handler()), but on the second. This is also important
for authentication, which cannot be performed on the first call.

Before:

% wget -O/dev/null -S https://localhost:19531/
--2012-11-28 18:29:43--  https://localhost:19531/
Resolving localhost (localhost)... 127.0.0.1
Connecting to localhost (localhost)|127.0.0.1|:19531... connected.
HTTP request sent, awaiting response...
  HTTP/1.1 301 Moved Permanently
  Connection: close
  Content-Length: 87
  Location: /browse
  Content-Type: text/html
  Date: Wed, 28 Nov 2012 17:29:44 GMT
Location: /browse [following]
--2012-11-28 18:29:43--  https://localhost:19531/browse
Connecting to localhost (localhost)|127.0.0.1|:19531... connected.
HTTP request sent, awaiting response...
  HTTP/1.1 200 OK
  Connection: close
  Content-Length: 23260
  Content-Type: text/html
  Date: Wed, 28 Nov 2012 17:29:44 GMT
Length: 23260 (23K) [text/html]

After:

% wget --no-check-certificate -O/dev/null -S https://localhost:19531/
--2012-11-28 18:30:05--  https://localhost:19531/
Resolving localhost (localhost)... 127.0.0.1
Connecting to localhost (localhost)|127.0.0.1|:19531... connected.
HTTP request sent, awaiting response...
  HTTP/1.1 301 Moved Permanently
  Content-Length: 87
  Location: /browse
  Content-Type: text/html
  Date: Wed, 28 Nov 2012 17:30:05 GMT
Location: /browse [following]
--2012-11-28 18:30:05--  https://localhost:19531/browse
Reusing existing connection to localhost:19531.
HTTP request sent, awaiting response...
  HTTP/1.1 200 OK
  Content-Length: 23260
  Content-Type: text/html
  Date: Wed, 28 Nov 2012 17:30:06 GMT
Length: 23260 (23K) [text/html]