Compact row versions

[Pekka Enberg]

As transactions commit, we keep creating new versions. But we need to compact the versions at some point to avoid running out of memory.

[Piotr Sarna]

@penberg is there any substantial difference between this issue and #2? When nobody is currently reading a particular version anymore, it should be garbage collected. Do we miss that tracking?

[Piotr Sarna]

Nvm!!! Just realized that this issue is about cleaning up the data structure that stores actual rows, and #2 was about tracking transaction metadata