Ensure asset coherency

[iboB]

This is blocked as it depends on #52 but adding issue so we don't forget

The adding of xxhash to the repo was part of this issue: alpaca-core/alpaca-core-sdk@eba6e2d

[iboB]

Moving this to post-MVP. Scope is reduced as part of alpaca-core/alpaca-core-sdk#139

[iboB]

This is now scoped as part of acord (which is a daemon, so it will be much easier and no sqlite will be needed)