optimization(download): Consider using splice on linux to download snapshots

[InKryption]

Currently in src/accountsdb/download.zig, we're downloading snapshots to disk using a cross-platform but naive approach, where our client reads the HTTP response body into memory, and then we write that data to the resulting file. This can result in a lot of context switching, since we're reading from kernelspace into userspace, and then writing from userspace back to kernelspace.

What we could instead do on linux is create a pipe, and splice the incoming response body through the pipe and then directly into the resulting file - this would avoid copying the data into userspace altogether.

Closing this issue will entail:

1. Make this change in a pull request.
2. Benchmark the performance difference.
3. Merge the change if it's a performance gain, or close it if it's a loss or a wash.