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.h

@@ -30,6 +30,8 @@ class WalletLoader;
 } // namespace interfaces
 
 namespace node {
+class KernelNotifications;
+
 //! NodeContext struct containing references to chain state and connection
 //! state.
 //!
@@ -62,6 +64,7 @@ struct NodeContext {
     interfaces::WalletLoader* wallet_loader{nullptr};
     std::unique_ptr<CScheduler> scheduler;
     std::function<void()> rpc_interruption_point = [] {};
+    std::unique_ptr<KernelNotifications> notifications;
 
     //! Declare default constructor and destructor that are not inline, so code
     //! instantiating the NodeContext struct doesn't need to #include class