Solver.getTimeMillisSpent() and Solver.getBestScore() need to be thread safe (and javadocced as such)

When calling getTimeMillisSpent from an other thread just at the start of solving you sometimes get a NullPointerException

Here is the relevant part of the stack trace, the calling thread just calls

solver.getTimeMillisSpent()

java.lang.NullPointerException
        at org.optaplanner.core.impl.solver.DefaultSolver.getTimeMillisSpent(DefaultSolver.java:116)

I haven't been able to make a reproducer since it is very intermittent, and hard to reproduce consistently.