Added lib/tlb.lua: Page-based address translation cache.

This can be used to cache page-based address translations, such as the
mappings between physical and virtual memory.

Modeled on a hardware Translation Lookaside Buffer:
http://en.wikipedia.org/wiki/Translation_lookaside_buffer
Use TLB cache (lib.tlb) in memory.virtual_to_physical().

Speeds up address translations. Measurable impact on loadgen
benchmark.
Merge pull request #205 from lukego/tlb2

Optimize memory.virtual_to_physical() with software TLB