tlsdated: exponential backoff in error cases.

If we're failing to get a tls connection, it may be because the remote host is
down or overloaded, in which case our current aggressive retry behavior is
counterproductive; instead, do exponential backoff each time a tlsdate request
fails.

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>
Merge pull request #64 from elly/backoff

tlsdated: exponential backoff in error cases.