Subtask: replace --save-blocks, --replay-blocks

[tbfleming]

[swatanabe]

I'd prefer to remove them.

• --save-blocks is unnecessary because it can be fully implemented via the WASM HTTP API. There's no good reason to block the main thread for it. It's also really inflexible to run it only at startup.
• --replay-blocks: I'd prefer to have this be psibase import <filename>. I also think that this should not be the recommended approach to creating a new node. If downloading the blocklog over p2p is significantly slower than downloading it via some other mechanism, we should ask ourselves why that is the case, and fix it.

[tbfleming]

Agreed