Integrate Configuration Framework with Tempesta modules

This commit includes the following changes:
- TfwCfgSpec implementation that allows to describe the configuration
  in a declarative manner.
- TfwCfgMod that allwos to register callbacks for start/stop events
- cfg_userspace_if.c containing sysctl interface for the configuration
- integration with tempesta modules: cache, client, server, session,
  connection, http, sock_frontend, sock_backend

Related to: #18 - Tempesta FW configuration framework