batch write sleep interval and provisioned capacity limits on data only restore

[simeonvandersteen]

I noticed that my restores were failing for two reasons:

• I did a --dataOnly restore, which doesn't update the capacity limits, so moved some code around to make that happen
• If you don't want to change the capacity limits for whatever reason, and the limits are too low, the batch write fails because it doesn't sleep between tries (I expected boto to do that but apparently it doesn't), I added a simple "linear sleep" which solves that (although it could be improved by using "exponential sleep" instead of linear)

These fixes work for me, I haven't tested it thoroughly. Hope it's useful!

[bchew]

@simeonvandersteen sorry for the late review on this, thanks for the PR!