Stronger memory constraints on server

[philip-davis]

Improve configurable memory constraints on server. Right now memory is controlled by 'max versions' only. Users might want better control that this. Some early thoughts:

Max versions should be configurable per variable
User might want to set memory headroom per process

Improving garbage collection comes along with this (we have to choose what to collect/evict)
-How many times do we expect data to be read?
-How granular should we be?
-Do we break up objects?

Soft deletion might be helpful too:
Eviction to disk?