General structure for UDP RX sessions

[serges147]

RX sessions functionality is not complete in this PR, but general structure is there. Msg RX session is implemented a bit further (but still not yet integrated with media sockets - will be the next PR).

• Added new implementation of a ScatteredBuffer::IStorage protocol - the UdpardMemory class manages (owns) scattered fragments received from libudpard.
• Added CAVL tree implementation - in use to ensure a single RX session per its kind & port id. New detail::SessionsTree wraps the tree by exposing only sessions relevant api, as well as managing tree node memory.

Also:

• minor fixes based on previous PR review