Rust: add discussion about what it means to "commit"

[brson]

Between project 2/3/4 we introduce writing to disk, the index, and returning a response over the network, and parallel updates. There's not really any discussion about what it means for data to be committed though, what sequence thing have to happen in, when and why disks need to be flushed.

In project 2 there is a commented-out section on basic database architecture, and that might be an appropriate place to put discussion about committing, but since each of 2/3/4 introduces topics that complicate the commit process I don't know offhand how I want to present the subject.