cmd/coordinator: fix potential deadlock in getStatus
cmd/coordinator: fix potential deadlock in getStatus

Map iteration is random, so it's possible for two simultaneous calls
to getStatus to deadlock due to the use of defer to unlock.

Change-Id: Ib2c0b4122bd5ea17fde5e4c60da1a853a12ed2fd
Reviewed-on: https://go-review.googlesource.com/40301
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build