[instance-locator] use instance reference instead of pointer (#2007)

This commit changes the `InstanceLocator` class to keep track of a
reference to `otInstance` (instead of a pointer) to make it behave
similar to other `ObjectLocator` classes. The method `GetInstance()`
in all locator objects is updated to provide a reference (instead
of a pointer) to `otInstance`.

The logging macros are updated such that a reference to `otInstance`
is passed as the first argument (with the exception of
`otLog<Level>Plat()` macros which are used by platform code in C
domain). The documentation for log macros are also updated.

src/cli/cli.cpp

@@ -244,7 +244,7 @@ Interpreter::Interpreter(otInstance *aInstance):
     mLength(8),
     mCount(1),
     mInterval(1000),
-    mPingTimer(aInstance, &Interpreter::s_HandlePingTimer, this),
+    mPingTimer(*aInstance, &Interpreter::s_HandlePingTimer, this),
 #if OPENTHREAD_ENABLE_DNS_CLIENT
     mResolvingInProgress(0),
 #endif