libmemif: introduce 'memif_per_thread_' namespace

libmemif: introduce 'memif_per_thread_' namespace

APIs in 'memif_per_thread_' namespace are used to split the global
database into separate databases, to improve multi-thread use cases.

Using 'memif_per_thread_init' client can create separate libmemif
databases (libmemif_main_t). Client will reference these databases
using memif_per_thread_handle_t. Each database requires unique socket.
Created interface will be stored in the same database as the socket
passed in connection arguments.

Example code: extras/libmemif/examples/icmp_responder_3-1/main.c

Type: feature

Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>
Change-Id: I261563ecc34761a76e94f20c20015394398ddfd7
Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>