Support starting `lbcd` with snapshot

[roylee17]

To help new nodes catch up faster without syncing blocks from scratch, we publish snapshots compressed in zstd format.

Some users requested a convenient way to start lbcd with the snapshot without manually downloading and uncompress the snapshot. This is handy for automated / integrated setup environment.

This can be done with:

1. a wrapper script which downloads and uncompresses the snapshot before starting lbcd.
2. --snapshot cli flag, which takes a file or url.
   Go has zstd package to do this.

Either implementation should support:

1. Download a snapshot from a URL
   Uncompress from a downloaded snapshot

2. Download and uncompress on the fly, for example:

curl https://snapshots.lbry.com/blockchain/lbcd_snapshot_1178238_v0.22.100_2022-06-17.tar.zst | zstd -d | tar xf - -C ${LBCD_DIR}

Note: 2) support of --snapshot is preferred so we don't require installing zstd packages