heartbeat for gsexit

ftaid of heartbeat from gsexit was set to the ftaid of the producing
fta. Now we set it to 0 as otherwise the clearinghouse will surpress
the heartbeat towards gshub.
use the app FTAID instead of 0
app annoucement

each app which uses the app library will announce as app to gshub
Merge pull request #74 from caskdata/feature/ping_from_gsexit

Merged https://github.com/caskdata/tigon/pull/74