feat: reduce default oct rows to discard from 3 to 2

[cryptonemo]

No description provided.

[cryptonemo]

Alternatively, we can do what we need to do without touching merkle_light, by just not using the default method here, and instead having our own default method in rust-fil-proofs (that uses the settings, etc)

[porcuquine]

I am approving this as the expedient way to change the default as desired for a short-term release. However, I think longer-term this default should not be set here. The 'right' default is dependent on usage and may change for reasons unrelated to the merkle_light implementation. (For example, if/when we optimize regenerating bottom tree segments, discarding more may make sense.)

[cryptonemo]

Closing in favor of filecoin-project/rust-fil-proofs#1165