refactor: Add NodeContext::shutdown member

Add NodeContext::shutdown variable and start using it to replace the
kernel::Context::interrupt variable. The latter can't easily be removed right
away but will be removed later in this PR.

Moving the interrupt object from the kernel context to the node context
increases flexibility of the kernel API so it is possible to use multiple
interrupt objects, or avoid creating one if one is not needed. It will also
allow getting rid of the kernel::g_context global later in this PR, replacing
it with a private SignalInterrupt instance in init.cpp

There is no change in behavior in this commit outside of unit tests. In unit
tests there should be no visible change either, but internally now each test
has its own interrupt variable so the variable will be automatically reset
between tests.

src/bitcoin-chainstate.cpp

@@ -125,7 +125,8 @@ int main(int argc, char* argv[])
         .blocks_dir = abs_datadir / "blocks",
         .notifications = chainman_opts.notifications,
     };
-    ChainstateManager chainman{kernel_context.interrupt, chainman_opts, blockman_opts};
+    util::SignalInterrupt interrupt;
+    ChainstateManager chainman{interrupt, chainman_opts, blockman_opts};
 
     node::CacheSizes cache_sizes;
     cache_sizes.block_tree_db = 2 << 20;