keep method UseRandomizedTTL thread safe

[peczenyj]

Hello

this method is not thread safe. I am not sure what was the intent with such method BTW

also, the LFU cache is already thread safe, it is not need add a mutex on the wrapper.

if it is possible remove this method at some point, there is no need for mutex

[peczenyj]

Hello

I continue some refactors and create the pull request #101 where I convert this method to a functional option

you can close this PR if you want @vmihailenco