ssa_plugin.c: Improve LFT change event handling

In case of existing LFT top/block changed record
new record is not being inserted instead but the old
record fields are updated with the new data.
This allows us to save additional lookups.

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