Stop using deprecated twisted.test.proto_helpers.MemoryReactor{,Clock}

[exarkun]

We use these two APIs in the test suite and they will be deprecated in the next Twisted release.

Update to prefer the new names but leave fallbacks to the old names since we won't _require_ a Twisted that has the new names for a while.

[exarkun]

Argh. This is trash. Tahoe-LAFS doesn't use these. Instead, Twisted itself, Autobahn, and treq use them:

.../site-packages/autobahn/twisted/testing/__init__.py:43: DeprecationWarning: twisted.test.proto_helpers.MemoryReactorClock was deprecated in Twisted NEXT: Please use twisted.internet.testing.MemoryReactorClock instead.
  from twisted.test.proto_helpers import MemoryReactorClock
.../site-packages/twisted/test/iosim.py:27: DeprecationWarning: twisted.test.proto_helpers.MemoryReactorClock was deprecated in Twisted NEXT: Please use twisted.internet.testing.MemoryReactorClock instead.
  from .proto_helpers import MemoryReactorClock
.../site-packages/treq/testing.py:13: DeprecationWarning: twisted.test.proto_helpers.MemoryReactor was deprecated in Twisted NEXT: Please use twisted.internet.testing.MemoryReactor instead.
  from twisted.test.proto_helpers import MemoryReactor

So those projects all need to implement fixes themselves. Perhaps the primary problem for Tahoe-LAFS here is that the Tahoe-LAFS "deprecations" CI job fails whenever any _third party_ library triggers a deprecation warning.

[exarkun]

​https://github.com/twisted/twisted/pull/1174 ​https://github.com/twisted/treq/pull/253

[exarkun]

​https://github.com/crossbario/autobahn-python/pull/1232

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/638

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/commit/d5f647d03e6de4abe9724e1ed4e029a3af90dbee