MAJOR: spoe: Add an experimental Stream Processing Offload Engine

MAJOR: spoe: Add an experimental Stream Processing Offload Engine

SPOE makes possible the communication with external components to retrieve some
info using an in-house binary protocol, the Stream Processing Offload Protocol
(SPOP). In the long term, its aim is to allow any kind of offloading on the
streams. This first version, besides being experimental, won't do lot of
things. The most important today is to validate the protocol design and lay the
foundations of what will, one day, be a full offload engine for the stream
processing.

So, for now, the SPOE can offload the stream processing before "tcp-request
content", "tcp-response content", "http-request" and "http-response" rules. And
it only supports variables creation/suppression. But, in spite of these limited
features, we can easily imagine to implement a SSO solution, an ip reputation
service or an ip geolocation service.

Internally, the SPOE is implemented as a filter. So, to use it, you must use
following line in a proxy proxy section:

  frontend my-front
      ...
      filter spoe [engine <name>] config <file>
      ...

It uses its own configuration file to keep the HAProxy configuration clean. It
is also a easy way to disable it by commenting out the filter line.

See "doc/SPOE.txt" for all details about the SPOE configuration.