validation: Move block into BlockConnected signal

This makes existing behaviour of the block's destructor triggering on
the scheduler thread more explicit by moving it to the thread. The
scheduler thread doing so is useful, since it does not block the thread
doing validation while releasing a block's memory.

Previously, both the caller and the queued event lambda held copies of
the shared_ptr. The block would typically be freed on the scheduler
thread - but only because it went out of scope before the queued event
on the scheduler thread ran. If the scheduler ran first, the block would
instead be freed on the validation thread.

Now, ownership is transferred at each step when invoking the
BlockConnected signal: connected_blocks yields via std::move,
BlockConnected takes by value, and the event lambda move-captures the
shared_ptr. Though it is possible that this only decrements the block's
reference count, blocks are also read from disk in `ConnectTip`, which
now explicitly results in their memory being released on the scheduler
thread.

src/validationinterface.cpp

@@ -219,12 +219,12 @@ void ValidationSignals::TransactionRemovedFromMempool(const CTransactionRef& tx,
     ENQUEUE_AND_LOG_EVENT(std::move(event), std::move(log_msg));
 }
 
-void ValidationSignals::BlockConnected(const ChainstateRole& role, const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindex)
+void ValidationSignals::BlockConnected(const ChainstateRole& role, std::shared_ptr<const CBlock> pblock, const CBlockIndex* pindex)
 {
     auto log_msg = LOG_MSG("%s: block hash=%s block height=%d", __func__,
                           pblock->GetHash().ToString(),
                           pindex->nHeight);
-    auto event = [role, pblock, pindex, this] {
+    auto event = [role, pblock = std::move(pblock), pindex, this] {
         m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.BlockConnected(role, pblock, pindex); });
     };
     ENQUEUE_AND_LOG_EVENT(std::move(event), std::move(log_msg));