perf(rpc server): Measure benefit of queuing snapshot & pipe opening as well

[InKryption]

Right now in the io_uring Workpool backend, when we begin processing a snapshot request, we inline open the relevant file and create the splicing pipe:

sig/src/rpc/server/linux_io_uring.zig

Lines 490 to 500 in 2086fd9

	const archive_name_bounded = snap_info.snapshotArchiveName();
	const archive_name = archive_name_bounded.constSlice();
	const snapshot_dir = server_ctx.snapshot_dir;
	const archive_file = try snapshot_dir.openFile(archive_name, .{});
	errdefer archive_file.close();
	const file_size = try archive_file.getEndPos();
	const pipe_r, const pipe_w = try std.posix.pipe();
	errdefer std.posix.close(pipe_w);
	errdefer std.posix.close(pipe_r);

This issue can be closed when a PR is created which refactors this into a set of SQE submissions and CQEs for opening the file and creating the pipe; after it's created, the complexity increase should be weighed against the performance benefit, and then it should be either merged or closed based on that evaluation.