[transfer-engine] Delay transfer engine exit

When transferring multiple files via Bluetooth, we quickly receive
one transfer request after another, so this causes a timing issue
where the engine may quit immediately after the first request and
is in the process of closing down when the next request is received,
and thus cannot be started again for the next request.
Merge pull request #17 from nemomobile/delayed-exit-safely

[transfer-engine] Delay transfer engine exit