NetworkConnector no longer waits 10 minutes before failing

Reduced the number of attempts from 10 to 5.

Because of the exponential backoff, the sixth attempt starts one minute after the first attempt and the tenth starts closer to ten minutes after.

If a call hasn't connected after a minute (5 attempts), it's reasonable to propagate failure to the user.
Waiting 17 minutes (10 attempts) is a bit much.
Most are unlikely to wait 20 seconds.

(An alternative change would be to lower the backoff factor.)
Merge pull request #116 from Strilanc/patch-6

NetworkConnector no longer waits 10 minutes before failing