Overwrite encrypted loopback files with random data to prevent meta information leaks

[BenWestgate]

Is your feature request related to a problem? Please describe.
For #81 it could give away which vaults have been used or not if they are not pre-filled with randomness.

Describe the solution you'd like
I'd like the files written with randomness so that it's not possible to tell how much space has been used.

Similarly if the USB stick has more space than the full blockchain will use, we should write random data to the free space after IBD to obscure the amount of space used. Otherwise it may be possible to infer the USB stick contains a Bitcoin node.

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