internal image registry was down due to data race

Issue

• After running several hours or days, image registry was suddenly restarted it's own. Below is the critical panic logs when issue happened:

  dockerd-current[79274]: fatal error: concurrent map writes
  dockerd-current[79274]:
  dockerd-current[79274]: goroutine 174282 [running]:
  dockerd-current[79274]: runtime.throw(0x19ac0f9, 0x15)
  dockerd-current[79274]:         /usr/lib/golang/src/runtime/panic.go:566 +0x95 fp=0xc425eab1a8 sp=0xc425eab188
  dockerd-current[79274]: runtime.mapassign1(0x1758e60, 0xc421d01b90, 0xc425eab390, 0xc425eab380)
  dockerd-current[79274]:         /usr/lib/golang/src/runtime/hashmap.go:458 +0x8ef fp=0xc425eab290 sp=0xc425eab1a8
  dockerd-current[79274]: github.com/openshift/origin/pkg/dockerregistry/server.(*remoteBlobGetterService).proxyStat(0xc421b6e300, 0x7f8e69351528, 0xc425785590, 0x2530f80, 0xc4228862d0, 0xc425eab808, 0xc425fce5a5, 0x47, 0x0, 0x0, ...)
  dockerd-current[79274]:         /builddir/build/BUILD/atomic-openshift-git-0.b6f55a2/_output/local/go/src/github.com/openshift/origin/pkg/dockerregistry/server/remoteblobgetter.go:181 +0xcf9 fp=0xc425eab720 sp=0xc425eab290
  dockerd-current[79274]: github.com/openshift/origin/pkg/dockerregistry/server.(*remoteBlobGetterService).findCandidateRepository(0xc421b6e300, 0x7f8e69351528, 0xc425785590, 0xc421addb90, 0x1, 0x1, 0xc4252d5b60, 0xc421c35900, 0x2, 0x2, ...)
  dockerd-current[79274]:         /builddir/build/BUILD/atomic-openshift-git-0.b6f55a2/_output/local/go/src/github.com/openshift/origin/pkg/dockerregistry/server/remoteblobgetter.go:228 +0x1ab fp=0xc425eab908 sp=0xc425eab720
  dockerd-current[79274]: github.com/openshift/origin/pkg/dockerregistry/server.(*remoteBlobGetterService).Stat(0xc421b6e300, 0x7f8e69351528, 0xc425785590, 0xc425fce5a5, 0x47, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
  dockerd-current[79274]:         /builddir/build/BUILD/atomic-openshift-git-0.b6f55a2/_output/local/go/src/github.com/openshift/origin/pkg/dockerregistry/server/remoteblobgetter.go:94 +0x3e1 fp=0xc425eabc38 sp=0xc425eab908
  dockerd-current[79274]: github.com/openshift/origin/pkg/dockerregistry/server.(*pullthroughBlobStore).copyContent(0xc425377760, 0x254fd00, 0xc421b6e300, 0x7f8e69351528, 0xc425785590, 0xc425fce5a5, 0x47, 0x7f8e692b1c78, 0xc4206b98a0, 0x0, ...)
  dockerd-current[79274]:         /builddir/build/BUILD/atomic-openshift-git-0.b6f55a2/_output/local/go/src/github.com/openshift/origin/pkg/dockerregistry/server/pullthroughblobstore.go:152 +0xa9 fp=0xc425eabd50 sp=0xc425eabc38
  dockerd-current[79274]: github.com/openshift/origin/pkg/dockerregistry/server.(*pullthroughBlobStore).storeLocal(0xc425377760, 0x254fd00, 0xc421b6e300, 0x7f8e69351528, 0xc425785590, 0xc425fce5a5, 0x47, 0x0, 0x0)
  dockerd-current[79274]:         /builddir/build/BUILD/atomic-openshift-git-0.b6f55a2/_output/local/go/src/github.com/openshift/origin/pkg/dockerregistry/server/pullthroughblobstore.go:199 +0x1d2 fp=0xc425eabe58 sp=0xc425eabd50
  dockerd-current[79274]: github.com/openshift/origin/pkg/dockerregistry/server.(*pullthroughBlobStore).ServeBlob.func1(0x7f8e69351528, 0xc425785590, 0xc425377760, 0x254fd00, 0xc421b6e300, 0xc425fce5a5, 0x47)
  dockerd-current[79274]:         /builddir/build/BUILD/atomic-openshift-git-0.b6f55a2/_output/local/go/src/github.com/openshift/origin/pkg/dockerregistry/server/pullthroughblobstore.go:85 +0x19f fp=0xc425eabf18 sp=0xc425eabe58