Provide random number generators without locks

[MBradbury]

It would be nice if the random number generators could be provided without locks. In my simulations there is a single RNG per thread, so the locks are simply overhead. Removing the locks gets me about a 20% speed improvement (in a naive test).

[slandelle]

Just to let you know: we have a benchmark here where we forked CellularAutomatonRNG and MersenneTwisterRNG to remove the ReentrantLocks and I confirm the performance boost (if of course you can afford ThreadLocals).