plugin/ssa_extract.c: Add ssa_db_extract_alloc_tbls method

Allocating node/lft_top/lft_block tables
moved into a new separate method - ssa_db_extract_alloc_tbls.

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