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/Makefile.am

@@ -186,6 +186,7 @@ BITCOIN_CORE_H = \
   kernel/mempool_limits.h \
   kernel/mempool_options.h \
   kernel/mempool_persist.h \
+  kernel/notifications_interface.h \
   kernel/validation_cache_sizes.h \
   key.h \
   key_io.h \
@@ -214,6 +215,7 @@ BITCOIN_CORE_H = \
   node/database_args.h \
   node/eviction.h \
   node/interface_ui.h \
+  node/kernel_notifications.h \
   node/mempool_args.h \
   node/mempool_persist_args.h \
   node/miner.h \
@@ -408,6 +410,7 @@ libbitcoin_node_a_SOURCES = \
   node/eviction.cpp \
   node/interface_ui.cpp \
   node/interfaces.cpp \
+  node/kernel_notifications.cpp \
   node/mempool_args.cpp \
   node/mempool_persist_args.cpp \
   node/miner.cpp \