libdash supports now persistent connections and pipelining

libdash supports now HTTP/1.1 persistent connections and pipelining. Moreover, the HTTPConnectionManager maintains a pool of connections that will be used in a round robin manner. Currently, we only use one per host but in the future the user will be able to control the number of connections dynamically. Persistent connections and pipelining are automatically enabled so you do not have to change anything at your existing applications.
The changeset is available here and contains some other minor fixes and a Statistics Manager that will be integrated into the interface soon.