* Use both a in-proc and out-of-proc pipenv lock
Turns out that flock alone is not sufficient to guarantee exclusive
access to a resource within a single process. To remedy this, a few
FileMutex type wraps both an in-proc mutex and an out-of-proc
file-backed mutex to achieve the goal of exclusive access to a resource
in both in-proc and out-of-proc scenarios.
This commit also uses this lock globally in the integration test
framework in order to globally serialize invocations of pipenv install.
* Remove merge markers
