fix: use a monotonic clock in memory storage

That should be more robust against system time update, e.g.
ntp or leap seconds.

setup.py

@@ -38,7 +38,7 @@
     url='https://github.com/falconry/token-bucket',
     license='Apache 2.0',
     packages=find_packages(exclude=['tests']),
-    install_requires=[],
+    install_requires=['monotonic_cffi'],
     setup_requires=['pytest-runner'],
     tests_require=['pytest'],
 )

token_bucket/storage.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import time
+import monotonic_cffi as monotonic
 
 from .storage_base import StorageBase
 
@@ -96,7 +96,7 @@ def replenish(self, key, rate, capacity):
 
             tokens_in_bucket, last_replenished_at = self._buckets[key]
 
-            now = time.time()
+            now = monotonic.monotonic()
 
             # NOTE(kgriffs): This will detect many, but not all,
             #   manifestations of the race condition. If a later
@@ -123,7 +123,7 @@ def replenish(self, key, rate, capacity):
             ]
 
         except KeyError:
-            self._buckets[key] = [capacity, time.time()]
+            self._buckets[key] = [capacity, monotonic.monotonic()]
 
     def consume(self, key, num_tokens):
         """Attempt to take one or more tokens from a bucket.