Send sample arrays instead of single samples over channels.

This is a start for chunking sample values together into arrays before sending/receiving them via channels. Right now, the whole flow from scrape -> tiered storage -> memory storage is array-chunked, but the visitor which takes care of dumping the memory storage to disk still handles single samples only. The visitor will be a bit different to handle, so I want to keep it out of this PR to not make it even larger.