kernel: Add notification interface

This commit is part of the libbitcoinkernel project and seeks to remove
the ChainstateManager's and, more generally, the kernel library's
dependency on interface_ui with options methods in this and the following
few commits. By removing interface_ui from the kernel library, its
dependency on boost is reduced to just boost::multi_index.

Define a new kernel notification class with virtual methods for
notifying about internal kernel events. Create a new file in the node
library for defining a function creating the default set of notification
methods such that these do not need to be re-defined all over the
codebase. As a first step, add a `blockTip` method, wrapping
`uiInterface.NotifyBlockTip`.

src/node/context.cpp

@@ -11,6 +11,7 @@
 #include <net.h>
 #include <net_processing.h>
 #include <netgroup.h>
+#include <node/kernel_notifications.h>
 #include <policy/fees.h>
 #include <scheduler.h>
 #include <txmempool.h>