rpc: make get_coinbase_tx_sum a restricted RPC

It's pretty slow and I/O intensive

src/rpc/core_rpc_server.h

@@ -122,7 +122,7 @@ namespace cryptonote
         MAP_JON_RPC_WE_IF("flush_txpool",        on_flush_txpool,               COMMAND_RPC_FLUSH_TRANSACTION_POOL, !m_restricted)
         MAP_JON_RPC_WE("get_output_histogram",   on_get_output_histogram,       COMMAND_RPC_GET_OUTPUT_HISTOGRAM)
         MAP_JON_RPC_WE("get_version",            on_get_version,                COMMAND_RPC_GET_VERSION)
-        MAP_JON_RPC_WE("get_coinbase_tx_sum",    on_get_coinbase_tx_sum,        COMMAND_RPC_GET_COINBASE_TX_SUM)
+        MAP_JON_RPC_WE_IF("get_coinbase_tx_sum", on_get_coinbase_tx_sum,        COMMAND_RPC_GET_COINBASE_TX_SUM, !m_restricted)
         MAP_JON_RPC_WE("get_fee_estimate",       on_get_per_kb_fee_estimate,    COMMAND_RPC_GET_PER_KB_FEE_ESTIMATE)
         MAP_JON_RPC_WE_IF("get_alternate_chains",on_get_alternate_chains,       COMMAND_RPC_GET_ALTERNATE_CHAINS, !m_restricted)
         MAP_JON_RPC_WE_IF("relay_tx",            on_relay_tx,                   COMMAND_RPC_RELAY_TX, !m_restricted)