ssa_db.[h c]: API update

A new structure was added "struct ssa_db" uniting separate
database tables and their definitions into a single structure.

Create / Init / Destroy methods were added as well and implemented
in shared/ssa_db.c.

The following steps have to be taken when creating new DB:
[1] "struct db_table_def def_tbl_arr[]" containing all DB tables defs.
[2] "struct db_dataset dataset_tbl_arr[]" containing all datasets
    (1 separate array for data tables datasets and another one for
     field tables datasets).
[3] "struct db_field_def field_tbl_arr[]" containing all data tables
    field definitions.
[4] "ssa_db_create()" and "ssa_db_init()" methods have to be
    sequentially called.

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