cmd/buildlet, cmd/coordinator: check build keys

Have the buildlet send build keys to the buildlet in reverse mode.
Rename Info to Status, as it will be used periodically by the
coordinator to check that the buildlet is OK.

And add a simple connection test.

Change-Id: Ic7285636c7818e2584e11555a8f5b5b66be30638
Reviewed-on: https://go-review.googlesource.com/8593
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
7 files changed