fix: assorted prod performance fixes

[mvadari]

High Level Overview of Change

This PR has several fixes:

• Fixes some issues with Buffer still being used in the app (even though the polyfill is no longer there)
• Fixes some issues with time calculations on ledgers
• Shrinks the max ledger count from 20 to 15 to save memory
• Optimizes the ledger and validation store to save memory on the home page

Context of Change

The recent high load on the ledger has been stretching the memory and performance limits of the Explorer.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

TypeScript/Hooks Update

N/A

Before / After

N/A

Test Plan

Fixes work locally. The optimizations shrink the object sizes by 25-30%.

[ckniffen]

Can you also update the Buffer usages in /src/rippled/NFTTransactions.ts, /src/rippled/lib/utils.js and src/index.html?

[JST5000]

LGTM