This massively simplifies transit locking behavior by pushing some
locking down to the Policy level, and embedding either a local or global
lock in the Policy depending on whether caching is enabled or not.
* helper/keysutil: Add a policy encrypted path storage
* Add vendored deps
* Fix spelling and paths that start with a /
* Add a key version template to change configure the ciphertext prefix
* Use big.Int for base58 instead of external lib
* Update go requirment to 1.10
* Add a version prefix cache
* Move logic to helper function
* Cache the template parts
* Add a storage prefix to policy
* Add an error if the policy passed in is nil
* Pull in the go1.10 version of the math/big package until we can update
