common/thread: Make Barrier's 'count' member non-const

While admirable as a means to ensure immutability, this has the
unfortunate downside of making the class non-movable. std::move cannot
actually perform a move operation if the provided operand has const data
members (std::move acts as an operation to "slide" resources out of an
object instance). Given Barrier contains move-only types such as
std::mutex, this can lead to confusing error messages if an object ever
contained a Barrier instance and said object was attempted to be moved.