Avoids making use of a global lock. Slighlty faster in benchmarks, but
for most cases construction time doesn't matter and is bottle necked on
reading input anyway.

benchstat master_gotip.bench fastrand_gotip.bench
name             old time/op  new time/op  delta
Filter_Reset-4   2.58µs ± 4%  2.61µs ±13%   ~     (p=0.841 n=5+5)
Filter_Insert-4  37.8ns ± 7%  35.9ns ± 0%   ~     (p=0.190 n=5+4)
Filter_Lookup-4  37.7ns ± 6%  37.6ns ± 3%   ~     (p=0.841 n=5+5)