Add new HTTP client class

It is based on the WorkerPool class which is already used for the job
queue and master daemon. Each request must be encapsulated in an
instance of HttpClientRequest, which will then be passed to
HttpClientManager for processing. Upon completion, the request object
contains the response or error message.

While not yet enabled, the HTTP client implementation has been designed
with HTTP/1.1 and keep-alive in mind. For now it only uses HTTP/1.0,
though.

HttpClientManager will likely need more changes when integrating with
the RPC layer. SSL will also be integrated in a second step.

Reviewed-by: iustinp, ultrotter