This commit adds the ability to run a subcommand through Litestream.
Shutting down the subcommand will cause Litestream to gracefully
shutdown. Litestream will forward interrupt signals and wait for
the subprocess to shutdown.
This commit changes the restore to download multiple WAL files to
the local disk in parallel while another goroutine applies those
files in order. Downloading & applying the WAL files in serial
reduces the total throughput as WAL files are typically made up of
multiple small files.
