so-5.5.18: Overloaded mchains and delayed/periodic messages delivery

Since v.5.5.18 there is a new behavior in delivery of delayed and periodic messages to mchains if these mchains are overloaded:

• if mchains is configured for waiting if is it full then this waiting is not preformed for delayed/periodic messages. It is because the sending of such messages is performed on context of timer thread. All operations on this context must be done as fast as possible. It means impossibility to wait for some time on overloaded mchain. It means that the overflow reaction for that mchain will be performed immediately;
• the so_5::mchain_props::overflow_reaction_t::throw_exception is replaced to so_5::mchain_props::overflow_reaction::drop_newest. It means a slient drop of delayed/peridic message without raising any exception. It is because the context of timer thread is a special context: no exceptions should be thrown here.