blockchain: keep alternative blocks in LMDB

Alternative blocks are cleared on startup unless --keep-alt-blocks
is passed on the command line

src/cryptonote_core/cryptonote_core.cpp

@@ -208,6 +208,11 @@ namespace cryptonote
     "is acted upon."
   , ""
   };
+  static const command_line::arg_descriptor<bool> arg_keep_alt_blocks  = {
+    "keep-alt-blocks"
+  , "Keep alternative blocks on restart"
+  , false
+  };
 
   //-----------------------------------------------------------------------------------------------
   core::core(i_cryptonote_protocol* pprotocol):
@@ -325,6 +330,7 @@ namespace cryptonote
     command_line::add_arg(desc, arg_prune_blockchain);
     command_line::add_arg(desc, arg_reorg_notify);
     command_line::add_arg(desc, arg_block_rate_notify);
+    command_line::add_arg(desc, arg_keep_alt_blocks);
 
     miner::init_options(desc);
     BlockchainDB::init_options(desc);
@@ -456,6 +462,7 @@ namespace cryptonote
     std::string check_updates_string = command_line::get_arg(vm, arg_check_updates);
     size_t max_txpool_weight = command_line::get_arg(vm, arg_max_txpool_weight);
     bool prune_blockchain = command_line::get_arg(vm, arg_prune_blockchain);
+    bool keep_alt_blocks = command_line::get_arg(vm, arg_keep_alt_blocks);
 
     boost::filesystem::path folder(m_config_folder);
     if (m_nettype == FAKECHAIN)
@@ -671,6 +678,9 @@ namespace cryptonote
     r = m_miner.init(vm, m_nettype);
     CHECK_AND_ASSERT_MES(r, false, "Failed to initialize miner instance");
 
+    if (!keep_alt_blocks && !m_blockchain_storage.get_db().is_read_only())
+      m_blockchain_storage.get_db().drop_alt_blocks();
+
     if (prune_blockchain)
     {
       // display a message if the blockchain is not pruned yet