Add freelru to gc overhead comparison

[rockdaboot]

This PR adds FreeLRU to the GC overhead comparison. FreeLRU is a GC-less and fast "exact" HashMap LRU implementation.

The PR is applied on top of #19, so bette rget that in first and I'll do a rebase then.

Results

$ go run caches_gc_overhead_comparison.go -cache freelru
Cache:              freelru
Number of entries:  20000000
Number of repeats:  50
Value size:         100
GC pause for startup:  165.204µs
GC pause for freelru: 1.464987ms