feat: set the inner cache limit via env (#33)

Description
---
Set the cache (the ones that is caching blocks) size limit.

Motivation and Context
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How Has This Been Tested?
---

What process can a PR reviewer use to test or verify this change?
---

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

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cache.js

@@ -1,5 +1,5 @@
 class Cache {
-  constructor(limit = 1000) {
+  constructor(limit) {
     this.limit = limit;
     this.cache = new Map();
   }
@@ -27,5 +27,7 @@ class Cache {
   }
 }
 
-var cache = new Cache();
+var cache = new Cache(
+  +process.env.TARI_EXPLORER_OLD_BLOCKS_CACHE_SETTINGS || 1000
+);
 module.exports = cache;