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.
* Test fixes in health_endpoint_test.go, agent_endpoint_test.go and rtt_test.go
* Don't reuse the same agent config in TestAgent_ReconnectConfigSettings
These would manifest in the HTTP output as Javascript nulls instead of
empty lists, so we had unintentionally changed the interface while
porting to the new state store. We added code to each HTTP endpoint to
convert nil slices to empty ones so they JSON-ify properly, and we added
tests to catch this in the future.
