mem: do not check for invalid socket ID

mem: do not check for invalid socket ID

We will be assigning "invalid" socket ID's to external heap, and
malloc will now be able to verify if a supplied socket ID is in
fact a valid one, rendering parameter checks for sockets
obsolete.

This changes the semantics of what we understand by "socket ID",
so document the change in the release notes.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>