Move the HTTP and DNS endpoints into the agent and control
their lifespan via the agent.
This removes the requirement to manage HTTP and DNS servers
indpendent of the agent since the agent is mostly useless
without an endpoint and the endpoints without the agent.
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.
Frank Schroeder