Kernel lookup api method (#3000)

* Kernel lookup api

* Min and max height parameters, scan backwards

* Return null instead of 404 if kernel not found

* Return TxKernel instead of TxKernelPrintable

* Update description of KernelHandler

api/src/types.rs

@@ -17,7 +17,7 @@ use std::sync::Arc;
 use crate::chain;
 use crate::core::core::hash::Hashed;
 use crate::core::core::merkle_proof::MerkleProof;
-use crate::core::core::KernelFeatures;
+use crate::core::core::{KernelFeatures, TxKernel};
 use crate::core::{core, ser};
 use crate::p2p;
 use crate::util;
@@ -699,6 +699,13 @@ pub struct OutputListing {
 	pub outputs: Vec<OutputPrintable>,
 }
 
+#[derive(Debug, Serialize, Deserialize, Clone)]
+pub struct LocatedTxKernel {
+	pub tx_kernel: TxKernel,
+	pub height: u64,
+	pub mmr_index: u64,
+}
+
 #[derive(Serialize, Deserialize)]
 pub struct PoolInfo {
 	/// Size of the pool