Move update_checkpoints() to a later stage

update_checkpoints() makes a few DNS requests and can take up to 20-30 seconds to complete (3-6 seconds on average). It is currently called from core::handle_incoming_block() which holds m_incoming_tx_lock, so it blocks all incoming transactions and blocks processing while update_checkpoints() is running. This PR moves it to until after a new block has been processed and relayed, to avoid full monerod locking.

src/cryptonote_core/cryptonote_core.cpp

@@ -1656,10 +1656,6 @@ namespace cryptonote
     if (((size_t)-1) <= 0xffffffff && block_blob.size() >= 0x3fffffff)
       MWARNING("This block's size is " << block_blob.size() << ", closing on the 32 bit limit");
 
-    // load json & DNS checkpoints every 10min/hour respectively,
-    // and verify them with respect to what blocks we already have
-    CHECK_AND_ASSERT_MES(update_checkpoints(), false, "One or more checkpoints loaded from json or dns conflicted with existing checkpoints.");
-
     block lb;
     if (!b)
     {