* Init `\health` rpc endpoint
* remove additional info from `\health` rpc endpoint
* Cleanup imports
* Added time threshold for health check
* Update rpc doc
* Remove unnecessary checks for blocktime creation lag
* Clean up of unnecessary config usage
Updates https://github.com/tendermint/tendermint/issues/693
* Adjusted Heartbeat.Copy to return nil on
trying to copy a nil value instead of panicking.
* Also documented that WriteSignBytes panics
if the Heartbeat is nil.