Make GC comparisons more realistic

[rockdaboot]

This change generates the keys and values in advance. This means the measurements of the GC impact of the cache algorithms is no longer tainted by key/value creation, which comes with a lot of heap allocations.

After

$ go run caches_gc_overhead_comparison.go -cache stdmap
Cache:              stdmap
Number of entries:  20000000
Number of repeats:  50
Value size:         100
GC pause for startup:  232.824µs
GC pause for stdmap: 5.49024ms

$ go run caches_gc_overhead_comparison.go -cache freecache
Cache:              freecache
Number of entries:  20000000
Number of repeats:  50
Value size:         100
GC pause for startup:  1.112992ms
GC pause for freecache: 26.927167ms

$ go run caches_gc_overhead_comparison.go -cache bigcache
Cache:              bigcache
Number of entries:  20000000
Number of repeats:  50
Value size:         100
GC pause for startup:  424.326µs
GC pause for bigcache: 29.979629ms