Avoid deadlock

Storage & blockchain locks need to be taken out atomically.  This makes
Blockchain and the crappy epee lock wrapper both implement Lockable so
that std::lock can be used to establish the locks.

contrib/epee/include/syncobj.h

@@ -112,6 +112,11 @@ namespace epee
       return m_section.try_lock();
     }
 
+    bool try_lock()
+    {
+      return tryLock();
+    }
+
     // to make copy fake
     critical_section& operator=(const critical_section& section)
     {

src/cryptonote_core/blockchain.cpp

@@ -4334,6 +4334,11 @@ void Blockchain::unlock()
   m_blockchain_lock.unlock();
 }
 
+bool Blockchain::try_lock()
+{
+  return m_blockchain_lock.tryLock();
+}
+
 bool Blockchain::for_all_key_images(std::function<bool(const crypto::key_image&)> f) const
 {
   return m_db->for_all_key_images(f);

src/cryptonote_core/blockchain.h

@@ -900,6 +900,7 @@ namespace cryptonote
 
     void lock();
     void unlock();
+    bool try_lock();
 
     void cancel();
 

src/cryptonote_core/stake_transaction_processor.cpp

@@ -283,7 +283,9 @@ void StakeTransactionProcessor::process_block(uint64_t block_index, const block&
 
 void StakeTransactionProcessor::synchronize()
 {
-  CRITICAL_REGION_LOCAL1(m_storage_lock);
+  std::unique_lock<epee::critical_section> storage_lock{m_storage_lock, std::defer_lock};
+  std::unique_lock<Blockchain> blockchain_lock{m_blockchain, std::defer_lock};
+  std::lock(storage_lock, blockchain_lock);
 
   uint64_t height = m_blockchain.get_current_blockchain_height();