Add caching to improive performance
Caching is being worked on in this branch:

- https://github.com/moneroexamples/onion-monero-blockchain-explorer/tree/try_lru_cache2

Currently it is planned to have 3 caches:

 - mempool, so that we dont parse json all the time (seems done)
 - tx context details (seems done)
 - tx details in blocks for the front page  (seems done) 

Now testing, refining, thinking what can be improved more, etc.