Class BufferCore and class Buffer should be movable

[ByLiZhao]

Currently, the class BufferCore is not movable because the existence of the destructor suppresses the compiler from auto-generating a move constructor for BufferCore. Since BufferCore has a mutex as a member variable, it is already non-copyable. This makes class BufferCore and class Buffer non-copyable and non-movable. This doesn't seem right.