Add more stricter or granular lock for AdaptiveSampler.IsSampled()

[black-adder]

https://github.com/uber/jaeger-client-go/blob/master/sampler.go#L310

AdaptiveSampler.IsSampled() is called by RemotelyControlledSampler.IsSampled(); however, the read lock doesn't prevent AdaptiveSampler from creating more samplers. Either change it to use a write lock or add more granular locking.