v.5.7.4

This is a minor update with two small additions.

Now delivery filter can be set for MPSC mbox (see #40), but not all custom implementations of MPSC mboxes can support that feature.

Now SObjectizer allows to set a custom direct mbox and an agent (see #39).

Some old error codes described in so_5/ret_code.hpp are now marked as deprecated (#44). They will be removed in some future version.

Additional details related to that release can be found on the corresponding page in the project Wiki.