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