Enhancement: Limit index file size

Before, restic would create a single new index file on `prune` or
`rebuild-index`, this may lead to memory problems when this huge index is
created and loaded again. We're now limiting the size of the index file, and
split newly created index files into several smaller ones. This allows restic
to be more memory-efficient.

https://github.com/restic/restic/pull/1584
https://github.com/restic/restic/issues/1412
https://github.com/restic/restic/issues/979
https://github.com/restic/restic/issues/526