Revert "SMAPIv2: avoid calling ourselves via the Unix domain socket"

Unfortunately bypassing the Unix domain socket also bypassed a
special 'rpc' function which (unlike all other rpc instances) contains
logic for dealing with the Redirect exception. We probably should
put the Redirect logic in some special global place eventually, rather
than in the middle of the migrate code.

This reverts commit 45562dff9aa61d2905695b2e8d307ccc92961a6f.