When we are mounting a lot of volumes, we frequently hit rate limits.

Reduce the frequency with which we poll the status; introduces a bit of
latency but probably matches common attach times pretty closely, and
avoids causing rate limit problems everywhere.

Also, we now poll for longer, as when we timeout, the volume is in an
indeterminate state: it may be about to complete.  The volume controller
can tolerate a slow attach/detach, but it is harder to tolerate the
indeterminism.

Finally, we ignore a sequence of errors in DescribeVolumes (up to 5 in a
row currently).  So we will eventually return an error, but a one
off-failure (e.g. due to rate limits) does not cause us to spuriously
fail.