Prevent multiple calls to -flush.

Flushes are performed as background tasks, but since the task ID
is stored as an instance variable, calling -flush twice in a row
obliterates the first task ID and allows only the second one to
be ended correctly. This results in iOS killing your application
because a background task has been running for too long.
Merge pull request #81 from jwatson/master

Prevent multiple calls to -flush.

thanks @jwatson