feat: allow keeping an index of utxos in memory

[tmpolaczyk]

Fix #2173

This PR adds a new option in witnet.toml storage.utxos_in_memory that defaults to false. When enabled, the node will keep a map of address to list of UTXOs in memory, which allows to implement fast getBalance and getUtxoInfo methods. With this option enabled a call to getBalance takes around 5ms, while the same call with this option disabled may take up to 5000ms. The expected memory usage in mainnet is around 500 MB.

[drcpu-github]

Seems to work just fine 👍 Would appreciate feedback from @drcpu-github, who I suspect that will be the main user for this feature for the time being!

I'll try and have a look by integrating it in the test version of my explorer.

[drcpu-github]

@tmpolaczyk I have been testing this branch in the test version of my explorer and it works like a charm! I don't really notice any slowdown in either address lookups in the explorer or the building of the list of all accounts with a non-zero balance.

[aesedepece]

Merged in as 2d497aad9e3c8fb27d431378ffa4917c351f5603