so-5.5.8: Simplification of usage of send-family functions with ad-hoc agents

Since v.5.5.8 there is a possibility for direct passing ad-hoc agent proxies to send-family functions. Just for comparision:

so_5::rt::agent_coop_t & coop = ...;
auto a = coop.define_agent(); // Ad-hoc agent created, variable 'a' is a proxy to the agent.

using namespace std::chrono;

// v.5.5.8 code
so_5::send_to_agent< some_message >( a, ...);
so_5::send_delaye_to_agent< some_message >( a, milliseconds(50), ...);
so_5::send_periodic_to_agent< some_message >( a, milliseconds(50), milliseconds(50), ...);

// Pre v.5.5.8 code
so_5::send_to_agent< some_message >( a.direct_mbox(), ...);
so_5::send_delaye_to_agent< some_message >( a.direct_mbox(), milliseconds(50), ...);
so_5::send_periodic_to_agent< some_message >( a.direct_mbox(), milliseconds(50), milliseconds(50), ...);