Inflated RAM usage

[soulseekah]

We practically double the RAM by copying everything from Redis into PHP RAM space.
Raised in #13 by @msigley

This is an issue in some cases where we're talking about maybe 10k objects?
Say the values are 128M in serialized data length they would take up roughly 128M in MySQL RAM space (maybe), another 128M in Redis and another 128M in every PHP process.

Needs some benchmark love. But how can we limit the mget amount, make the mget more intelligent? Maybe keep a stack of LRU keys per URL?