Use -blocksdir after bitcoind 0.17 release

[Sjors]

Should be a matter of setting -blocksdir=/mnt/magnetic/blocks.

For newly provisioned machines, the commands that create the two symlinks can be removed (from ~/.bitcoin/blocks to /mnt/magnetic/blocks and from /mnt/magnetic/blocks to ~/.bitcoin/blocks-index).

For existing machines, the symlinks themselves can be safely deleted, after which blocks-index needs to be moved to blocks/index.