/cc @armon - This is a reasonably major refactor that I think cleans up
a lot of the logic with secrets in responses. The reason for the
refactor is that while implementing Renew/Revoke in logical/framework I
found the existing API to be really awkward to work with.
Primarily, we needed a way to send down internal data for Vault core to
store since not all the data you need to revoke a key is always sent
down to the user (for example the user than AWS key belongs to).
At first, I was doing this manually in logical/framework with
req.Storage, but this is going to be such a common event that I think
its something core should assist with. Additionally, I think the added
context for secrets will be useful in the future when we have a Vault
API for returning orphaned out keys: we can also return the internal
data that might help an operator.
So this leads me to this refactor. I've removed most of the fields in
`logical.Response` and replaced it with a single `*Secret` pointer. If
this is non-nil, then the response represents a secret. The Secret
struct encapsulates all the lease info and such.
It also has some fields on it that are only populated at _request_ time
for Revoke/Renew operations. There is precedent for this sort of
behavior in the Go stdlib where http.Request/http.Response have fields
that differ based on client/server. I copied this style.
All core unit tests pass. The APIs fail for obvious reasons but I'll fix
that up in the next commit.
