spanner: fix race in mock server
spanner: fix race in mock server

Lock access  to the "next" field to prevent a data race.

Change-Id: I79288105abd125a806096cc2db04adb0c3a4f50f
Reviewed-on: https://code-review.googlesource.com/31010
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go