This adds a distributed lock managed in Redis around all new
design management version creations, in order to prevent concurrent
commits to master in the design repository for each project.

The lock is acquired scoped to the project ID, meaning different
projects can take different locks concurrently, but design
uploads/deletions in the same repository block each other. Each mutation
is given 5 seconds of exclusive lock time to run, and an exponential
back-off is used to handle backpressure.

ExclusiveLeaseHelpers is modified to support exponential back-off