plugin: Adding transferring extracted SMDB from CORE to AL

The mechanism is working as following:

 - On SUBNET UP event SMDB is being extracted and stored

 - SSA_DB_UPDATE message is sent to downstream thread

 - When a new client (AL) is connected to downstream thread,
   the SMDB previously extracted is being transferred
   (the case of AL connected but no SMDB extracted yet
    will be handled in subsequent patch)

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>
core.c: In core_extract_handler, don't initialize or poll extractdown fds

These sockets are currently unidirectional and no messages are sent
in this direction.

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
core.c: Only ssa_extract_send_db_update when CORE_INTEGRATION not defined

Signed-off-by: Hal Rosenstock <hal@mellanox.com>