Create serial access interceptor

[codenrhoden]

Create a new gRPC interceptor that provides local/global volume locking to ensure serial volume access. Local locking is the default behavior, used when etcd is not present, using an in-memory map. Global/distributed locking is used when an etcd endpoint is provided.

This interceptor will deprecate the idempotency interceptor, which should now be achieved in individual plugins' implementations.