core.c: adding ssa_core_send() method

'ssa_core_send()' is used to send messages from
core to extract thread.

The argument passed is the message that is being sent.
The following messages are currently available:
 - SSA_DB_START_EXTRACT
 - SSA_DB_LFT_CHANGE
 - SSA_DB_EXIT

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