feat: add cache settings (#24)

Description
---
Add cache settings. 2 minutes on blocks, and other searches, 0 on
mempool. We can discuss the cache length. Once we go live, we can cache
the block after some time indefinitely (the ones that are never going to
be reorg again).

Motivation and Context
---

How Has This Been Tested?
---
Running tari-explorer locally. 

What process can a PR reviewer use to test or verify this change?
---
Run locally to make sure that the response headers are properly set.

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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routes/blocks.js

@@ -34,6 +34,7 @@ function fromHexString(hexString) {
 }
 
 router.get("/:height_or_hash", async function (req, res) {
+  res.setHeader("Cache-Control", "public, max-age=120, immutable");
   try {
     let client = createClient();
     let height_or_hash = req.params.height_or_hash;

routes/mempool.js

@@ -7,6 +7,7 @@ var router = express.Router();
 
 /* GET mempool page. */
 router.get("/:excessSigs", async function (req, res) {
+  res.setHeader("Cache-Control", "public, max-age=0, must-revalidate");
   try {
     let client = createClient();
     let txId = req.params.excessSigs.split("+");

routes/search_commitments.js

@@ -26,6 +26,7 @@ var express = require("express");
 var router = express.Router();
 
 router.get("/", async function (req, res) {
+  res.setHeader("Cache-Control", "public, max-age=120, immutable");
   let client = createClient();
   let commitments = (
     req.query.comm ||

routes/search_kernels.js

@@ -26,6 +26,7 @@ var express = require("express");
 var router = express.Router();
 
 router.get("/", async function (req, res) {
+  res.setHeader("Cache-Control", "public, max-age=120, immutable");
   let client = createClient();
   let nonces = (req.query.nonces || "").split(",");
   let signatures = (req.query.signatures || "").split(",");