Use BatchCreateSessions for Spanner session creation

[data-sync-user]

Per Google’s recommendation in https://console.cloud.google.com/support/cases/detail/v2/25438594?invt=Ab4mzQ&project=moz-fx-sync-prod-3f0c&inv=1 we should create multiple sessions in one call via Spanner’s BatchCreateSessions api call, as session creation in Spanner is expensive and can potentially contribute to Spanner instability, particularly when there’s a spike of such creation.

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