Bug 963424 - Part 1: let Sync API_VERSION be 1.5.
Bug 963424 - Part 2: use clusterURL more opaquely in SyncConfiguration.
Bug 963424 - Part 3: only persist cluster URL to prefs in Sync11Configuration.
Bug 963424 - Part 4: eliminate GlobalSession.API_VERSION, adjust ClientRecordTerminator to match.
Review comment: extract Sync11Configuration.
Bug 963424 - Use token server-provided cluster URL. r=nalexander