get_output_key method is commonly used when working with txs and their key images. Because the method is not const, passing blockchain object though const& or pointers to const is not possible in this context. This is especially problematic in external projects (e.g., projects in moneroexamples) that use monero C++ api to operate on the blockchain and txs.
Thus, having get_output_key method will simplify moving blockchain object around through const references and pointers to const objects.
Only for pre rct for obvious reasons.
Note: DO NOT use a known spent list which includes outputs
which are not known spent. If the list includes any output
that's just strongly thought to be spent, but not provably
so, you risk finding yourself unable to sync past the point
where that output is spent.
I estimate only 200 MB saved on current mainnet though,
unless the new blackballing rule unearths a good amount of
large-amount-set extra spent outs.
