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 adds two goroutines to perform autopilot tasks on the leader - one
to monitor the health of servers and another to periodically clean up
dead servers with a limit on removal count. Also adds a new http endpoint,
`/v1/operator/autopilot/health`, for querying this information through an
operator RPC endpoint.
Frank Schroeder