[PATCH] SMP SNMP

The attached patch implements aggregation of SNMP responses, similar to
how we aggregate some cache manager stats.

The code contains changes that allow us to share some of the classes
between Cache Manager and SNMP code:

   * implement the following base classes under the ipc directory/module:
     - Ipc::Forwarder (ipc/Forwarder{.cc,.h} files)
     - Ipc::Inquirer (ipc/Inquirer{.cc,.h} files)
     - Ipc::Request (ipc/Request{.cc,.h} files)
     - Ipc::Response (ipc/Response{.cc,.h} files)

   * fix the Mgr::Forwarder, Mgr::Inquirer, Mgr::Request and
Mgr::Response classes to be implemented as kid classes of the equivalent
ICP::* classes.

Also implements for the SNMP the same mechanism used for cache manager:
  The SNMP requests forwarder to coordinator which collects the
statistics from kids and aggregate them.

This is a Measurement Factory project.