cmd/coordinator: use Go 1.10
cmd/coordinator: use Go 1.10

No particular reason, other than using the latests to find any bugs.
And Go 1.8 is no longer supported. I guess we skipped Go 1.9.

Change-Id: I9a342c226ae53c7cc28a4b8e75b8d8e7834da917
Reviewed-on: https://go-review.googlesource.com/95575
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

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