Allow SvObjectSocket to consume arbitrary data.

Created by: portnov

Addressed problem description

Was discussed in #2058 (closed).

Solution description

• In DefaultImplicitConversionPolicy, introduced classmethod get_lenient_socket_types
• Default implementation returns StringsSocket and SvObjectSocket. Child classes can override this.

Preflight checklist

Put an x letter in each brackets when you're done this item:

• Code changes complete.
• Code documentation complete.
• Documentation for users complete (or not required, if user never sees these changes).
• Manual testing done.
• Unit-tests implemented.
• Ready for merge.