When running the import, currently blocks are loaded in batches into a
`seq` then passed to the importer as such.
In reality, blocks are still processed one by one, so the batching does
not offer any performance advantage. It does however require that the
client wastes memory, up to several GB, on the block sequence while
they're waiting to be processed.
This PR introduces a persister that accepts these potentially large
blocks one by one and at the same time removes a number of redundant /
unnecessary copies, assignments and resets that were slowing down the
import process in general.
7bbb0f4421