This patch removes duplicate internal copies of constants in the structs
package which are also defined in the api package. The api.KVOp type
with all its values for the TXN endpoint and the api.HealthXXX constants
are now used throughout the codebase.
This resulted in some circular dependencies in the testutil package
which have been resolved by copying code and constants and moving the
WaitForLeader function into a separate testrpc package.
This isn't needed/used yet, but it's a good hook to get in there so we
can add more atomic operations in the future. The Go API hides this detail
so that feels like a KV-specific API. The implications on the REST API are
pretty minimal.
