Support docker local store pull same image simultaneously

The current local store implements get() using the local puller. For all requests of pulling same docker image at the same time, the local puller just untar the image tarball as many times as those requests are, and cp all of them to the same directory, which wastes time and bear high demand of computation. We should be able to support the local store/puller only do these for the first time, and the simultaneous pulling request should wait for the promised future and get it once the first pulling finishes.