When composer exits, it doesn't wait for the manifest generation goroutines
to finish. This is generally a bad practice so let's introduce a bit of
syncing and a new Shutdown method to prevent this.
This also prevents the manifest generation goroutine from creating weird
states when interrupted on a random line of code.
Signed-off-by: Ondřej Budai <ondrej@budai.cz>
I think it's a natural fit, they access some fields from the server so let's
find a new home for them there.
Signed-off-by: Ondřej Budai <ondrej@budai.cz>
