Dynamic trailing computation

[marco6]

This PR adds a non-default strategy to dynamically compute the amount of entries to keep in memory based on the size of the snapshot.

The problem with using a fixed number is that each entry has a different size (depending on the number of pages written for each transaction). As such, the memory used (and then streamed to a lagging follower) might be way bigger than the size of the snapshot itself (even by order of magnitude).