Make RSpec::Mocks::Space more thread-safe.

It used to be possible for different threads to receive different proxy
objects, potentially a source of bugs such as #380.

This is non-trivial to write a spec for, but you can trivially
demonstrate the problem by adding a `sleep` into the fetch blocks, and
then running:

    o = Object.new
    t = Thread.new do
      RSpec::Mocks.space.proxy_for(o)
    end
    b = RSpec::Mocks.space.proxy_for(o)
    a = t.value

    expect(a).to eq(b)