Add \health rpc endpoint (#1306)

* 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

rpc/core/health.go

@@ -0,0 +1,30 @@
+package core
+
+import (
+	ctypes "github.com/tendermint/tendermint/rpc/core/types"
+)
+
+// Get node health. Checks whether new blocks are created.
+//
+// ```shell
+// curl 'localhost:46657/health'
+// ```
+//
+// ```go
+// client := client.NewHTTP("tcp://0.0.0.0:46657", "/websocket")
+// result, err := client.Health()
+// ```
+//
+// > The above command returns JSON structured like this:
+//
+// ```json
+// {
+// 	"error": "",
+// 	"result": {},
+// 	"id": "",
+// 	"jsonrpc": "2.0"
+// }
+// ```
+func Health() (*ctypes.ResultHealth, error) {
+	return &ctypes.ResultHealth{}, nil
+}