cryptonote_protocol_handler: sync speedup

A block queue is now placed between block download and
block processing. Blocks are now requested only from one
peer (unless starved).

Includes a new sync_info coommand.

tests/core_proxy/core_proxy.cpp

@@ -185,6 +185,19 @@ bool tests::proxy_core::handle_incoming_tx(const cryptonote::blobdata& tx_blob,
     return true;
 }
 
+bool tests::proxy_core::handle_incoming_txs(const std::list<blobdata>& tx_blobs, std::vector<tx_verification_context>& tvc, bool keeped_by_block, bool relayed, bool do_not_relay)
+{
+    tvc.resize(tx_blobs.size());
+    size_t i = 0;
+    for (const auto &tx_blob: tx_blobs)
+    {
+      if (!handle_incoming_tx(tx_blob, tvc[i], keeped_by_block, relayed, do_not_relay))
+          return false;
+      ++i;
+    }
+    return true;
+}
+
 bool tests::proxy_core::handle_incoming_block(const cryptonote::blobdata& block_blob, cryptonote::block_verification_context& bvc, bool update_miner_blocktemplate) {
     block b = AUTO_VAL_INIT(b);