exp/lighthorizon: Add parameters to preload ledger cache.

[Shaptic]

What

This adds two new command line parameters:

• --ledger-cache-preload=<count>, which starts a goroutine in the background to fill the cache with count ledgers
• --ledger-cache-preload-start=<ledger>, which specifies the ledger to start building the cache at. By default, it will just count backwards from the latest ledger stored at the ledger source.

Why

#4526

Known limitations

n/a