libmemif: refactor connection establishment

libmemif: refactor connection establishment

per_thread_ namespace fuctionality replaced by memif socket.
Interfaces are grouped by memif socket which holds interface database.
Each thread can create it's unique memif socket. The path name
can be equal across threads so that the app only uses one
UNIX socket. In case of listener socket, listener fd
can be obtained and set using APIs.

This change allows:
  - No lookup on file descriptor events
    - improves interrupt handling
  - Loopback support (connect two interfaces in one app)
    - usefull for debugging and testing
  - Improves code readability by providing control channel
    abstraction for each interface and listener sockets

Type: refactor

Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>
Change-Id: I1b8261042431c0376646ab4c4c831f6e59dd3eed