table: use Path as API instead of destination

ProcessPaths and DeletePathsforPeer API uses both Path and Destination
for outside. There is no good reason to use Destination. Let's use
Path.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
table: add helper functions to modify PathAttributes for 4 and 2 byte AS.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>