Share a fake pull request database across connections

Because connections can be recreated, ensure that the fake pull
request database (really a dictionary) is shared across instances
of connections and fake github classes.

Also, move the fake github3 classes to their own file -- they were
getting larger and unruly.

Change-Id: I471c1487039c8b25a0bab95d918f31b92b9cd32b
2 files changed