Pre-fetch transactions from the database

[sug0]

Clients performing shielded sync will want to fetch a batch of masp transactions between two block heights $[h_0, h_1]$. However, the current clients only perform requests in small batches (of around 30 blocks, and $30 \ll h_1 - h_0 + 1$). Perhaps we could pre-fetch and cache transfers from a higher block range, to decrease the number of calls to Postgres, and speed-up shielded sync. The extra data would be cached and eventually released from memory.