add method Counter

[recoilme]

return unique int, for use like counter
Counter(file,key) (uint32?,error?)

for speedup set+sync on every val%1000 == 0 - val+1000
for example on 0 value - set 1000
all increment - in memory
in case of crash os - value will be 1000 after crash
on val%1000 ==0 set 2000 and so on

may be mute error with 0 - for simplify in this case:
Set(file,Counter(file,key),value)

may be call Counter silently in Set with params Counter(file,file) in case of key is nil
so Set(file,nil,val) - will be autoincrement

[recoilme]

Looks good: https://github.com/recoilme/slowpoke/tree/counter
Test counter on crash may be?