Remove the ActionCache in V3

[edbaunton]

For V3, I think it is completely possible to implement the ActionCache in terms of the Asset API. This would allow it to be more flexible and less specialized.

To maintain the performance characteristics as pointed out by @sstriker, we would need to probably add support for inlining of blobs/trees into the responses returned from the Asset API so as to not introduce another hop to the CAS.

[erikmav]

Do you have a sketch of what parts of ActionCache would map to under the Asset API?

[EdSchouten]

An alternative would have been to fold the Remote Asset API into REvX. The Action message could have contained a oneof { Command; Fetch }.