Cleanup CollectionReadBuffers

[tmadlener]

This is a follow up to #502 which implemented a quick fix. Ideally the CollectionReadBuffers are refactored to a (move-only) type that clearly indicates the ownership of the contained buffers. Something like a unique_ptr with a custom (type-erased) deleter should work in this case, since we can populate the deleters at generation time.

[tmadlener]

Enabling AddressSanitizer also reveals that there are a couple of indirect leaks in the whole machinery at the moment that could be related to this.