kernel: Remove StartShutdown calls from validation code

This change drops the last kernel dependency on shutdown.cpp. It also adds new
hooks for libbitcoinkernel applications to be able to interrupt kernel
operations when the chain tip changes.

This is a refactoring that does not affect behavior. (Looking at the code it
can appear like the new break statement in the ActivateBestChain function is a
change in behavior, but actually the previous StartShutdown call was indirectly
triggering a break before, because it was causing m_chainman.m_interrupt to be
true. The new code just makes the break more obvious.)

src/validation.h

@@ -23,7 +23,6 @@
 #include <policy/packages.h>
 #include <policy/policy.h>
 #include <script/script_error.h>
-#include <shutdown.h>
 #include <sync.h>
 #include <txdb.h>
 #include <txmempool.h> // For CTxMemPool::cs
@@ -970,7 +969,6 @@ public:
     const arith_uint256& MinimumChainWork() const { return *Assert(m_options.minimum_chain_work); }
     const uint256& AssumedValidBlock() const { return *Assert(m_options.assumed_valid_block); }
     kernel::Notifications& GetNotifications() const { return m_options.notifications; };
-    int StopAtHeight() const { return m_options.stop_at_height; };
 
     /**
      * Alias for ::cs_main.