TransactionPool uses non-dummy chain trait

Introduced new non-dummy trait for the blockchain as seen from the
pool that just produces a UTXO. Made to pool parametric on that
trait to get rid of the Box wrapper and still allow the test
implementation.

pool/src/types.rs

@@ -34,8 +34,6 @@ use core::core::transaction;
 use core::core::block;
 use core::core::hash;
 
-
-
 /// Placeholder: the data representing where we heard about a tx from.
 ///
 /// Used to make decisions based on transaction acceptance priority from 
@@ -84,6 +82,15 @@ pub enum PoolError {
     OrphanTransaction,
 }
 
+/// Interface that the pool requires from a blockchain implementation.
+pub trait BlockChain {
+  /// Get an unspent output by its commitment. Will return None if the output
+  /// is spent or if it doesn't exist. The blockchain is expected to produce
+  /// a result with its current view of the most worked chain, ignoring
+  /// orphans, etc.
+  fn get_unspent(&self, output_ref: &Commitment) -> Option<transaction::Output>;
+}
+
 /// Pool contains the elements of the graph that are connected, in full, to
 /// the blockchain.
 /// Reservations of outputs by orphan transactions (not fully connected) are