Clean up CoAP CLI. (#1864)

- Change static methods into class methods.
- Remove client/server distinction.
- Fix callback method types and remove unnecessary type casts.
- Update README.md to reflect changes.

src/cli/cli.cpp

@@ -223,6 +223,9 @@ typedef otPtr<const uint8_t> otBufferPtr;
 typedef otPtr<const char> otStringPtr;
 
 Interpreter::Interpreter(otInstance *aInstance):
+#if OPENTHREAD_ENABLE_APPLICATION_COAP
+    mCoap(*this),
+#endif
     mServer(NULL),
 #ifdef OTDLL
     mApiInstance(otApiInit()),
@@ -313,7 +316,7 @@ int Interpreter::Hex2Bin(const char *aHex, uint8_t *aBin, uint16_t aBinLength)
     return static_cast<int>(cur - aBin);
 }
 
-void Interpreter::AppendResult(otError aError)
+void Interpreter::AppendResult(otError aError) const
 {
     if (aError == OT_ERROR_NONE)
     {
@@ -325,7 +328,7 @@ void Interpreter::AppendResult(otError aError)
     }
 }
 
-void Interpreter::OutputBytes(const uint8_t *aBytes, uint8_t aLength)
+void Interpreter::OutputBytes(const uint8_t *aBytes, uint8_t aLength) const
 {
     for (int i = 0; i < aLength; i++)
     {
@@ -657,7 +660,7 @@ exit:
 void Interpreter::ProcessCoap(int argc, char *argv[])
 {
     otError error;
-    error = Coap::Process(mInstance, argc, argv, *mServer);
+    error = mCoap.Process(argc, argv);
     AppendResult(error);
 }