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)

benchmarks/thread_safety.rb

@@ -0,0 +1,24 @@
+$LOAD_PATH.unshift(File.expand_path("../../lib", __FILE__))
+
+require 'benchmark'
+require 'rspec/mocks'
+
+Benchmark.bm do |bm|
+  bm.report("fetching a proxy") do
+    RSpec::Mocks.with_temporary_scope do
+      o = Object.new
+      100000.times {
+        RSpec::Mocks.space.proxy_for(o)
+      }
+    end
+  end
+end
+
+# Without synchronize (not thread-safe):
+#
+#       user     system      total        real
+# fetching a proxy  0.120000   0.000000   0.120000 (  0.141333)
+#
+# With synchronize (thread-safe):
+#       user     system      total        real
+# fetching a proxy  0.180000   0.000000   0.180000 (  0.189553)

lib/rspec/mocks/space.rb

@@ -48,6 +48,8 @@ def initialize
         @any_instance_recorders  = {}
         @constant_mutators       = []
         @expectation_ordering    = OrderGroup.new
+        @proxy_mutex             = new_mutex
+        @any_instance_mutex      = new_mutex
       end
 
       def new_scope
@@ -75,9 +77,11 @@ def constant_mutator_for(name)
       end
 
       def any_instance_recorder_for(klass)
-        id = klass.__id__
-        any_instance_recorders.fetch(id) do
-          any_instance_recorder_not_found_for(id, klass)
+        any_instance_mutex.synchronize do
+          id = klass.__id__
+          any_instance_recorders.fetch(id) do
+            any_instance_recorder_not_found_for(id, klass)
+          end
         end
       end
 
@@ -86,8 +90,10 @@ def proxies_of(klass)
       end
 
       def proxy_for(object)
-        id = id_for(object)
-        proxies.fetch(id) { proxy_not_found_for(id, object) }
+        proxy_mutex.synchronize do
+          id = id_for(object)
+          proxies.fetch(id) { proxy_not_found_for(id, object) }
+        end
       end
 
       alias ensure_registered proxy_for
@@ -98,6 +104,21 @@ def registered?(object)
 
     private
 
+      attr_reader :proxy_mutex, :any_instance_mutex
+
+      # We don't want to depend on the stdlib ourselves, but if the user is
+      # using threads then a Mutex will be available to us. If not, we don't
+      # need to synchronize anyway.
+      def new_mutex
+        (defined?(::Mutex) ? ::Mutex : FakeMutex).new
+      end
+
+      class FakeMutex
+        def synchronize
+          yield
+        end
+      end
+
       def proxy_not_found_for(id, object)
         proxies[id] = case object
           when NilClass   then ProxyForNil.new(@expectation_ordering)