`SyncTransactions` follow-ups

[SantiagoPittella]

After #1224 some follow up took place:

1. There are some inconsistencies in the field's name for TransactionHeader proto message, e.g. the input_notes field is just nullifiers.
2. After retrieving the txs, we make N queries to get the associated notes. We should should be merging into a single query.
3. We should collect the size assumptions for the paginated queries into a single module, were we can be more consistent between endpoint. Also, we should set 4MB as the payload limit for all endpoints.
4. Rename transaction_records -> transaction in the proto message.
5. Rename transaction_header -> header in the proto message.