Implement UTXO tracking & double-spend protection

The UtxoSet class uses a hash-map for its hot cache,
and an SQLite database for its cold store.

Outputs which are the coldest are periodically moved
from the hot cache to the cold store.

On shutdown, all the items are stored.

source/agora/common/Config.d

@@ -95,6 +95,9 @@ public struct NodeConfig
     /// The seed to use for the keypair of this node
     public immutable KeyPair key_pair;
 
+    /// Maximum number of UTXOs to keep in the hot cache (for performance)
+    public size_t utxo_cache_size = 4096;
+
     /// Number of msecs to wait before retrying failed connections
     public long retry_delay = 3000;
 
@@ -270,6 +273,10 @@ private NodeConfig parseNodeConfig (Node node)
     auto max_listeners = node["max_listeners"].as!size_t;
     auto address = node["address"].as!string;
 
+    size_t utxo_cache_size = NodeConfig.utxo_cache_size.init;
+    if (auto delay = "utxo_cache_size" in node)
+        utxo_cache_size = delay.as!size_t;
+
     long retry_delay = 3000;
     if (auto delay = "retry_delay" in node)
         retry_delay = cast(long)(delay.as!float * 1000);
@@ -292,6 +299,7 @@ private NodeConfig parseNodeConfig (Node node)
         port : port,
         key_pair : key_pair,
         retry_delay : retry_delay,
+        utxo_cache_size : utxo_cache_size,
         max_retries : max_retries,
         timeout : timeout,
         data_dir : data_dir,