Server Accepts Huge Txns (e.g. 70mb) then Hits Java Heap Error

[aaj3f]

Description

Currently, there are no limits on maximum transaction size accepted by server, but with large enough transactions, the transaction processing hits a fatal Java heap error.

I'll leave it to the team to consider how to resolve this, whether it's establishing an arbitrary threshold for transaction sizes, or trying to handle heap allocation more safely.

This ticket is somewhat related to https://github.com/fluree/core/issues/41 as we're currently forced into a pattern of submitting single large txns rather than batched, sequential txns