Fix session bead adoption race

[julianknutsen]

Summary

• serialize adoption barrier session bead creation with the city session identifier lock
• re-check for a live session bead inside the lock before creating
• pass cityPath into startup/controller adoption paths
• add regression coverage for the adoption/sync lock race

Fixes #1155

Tests

• go test ./cmd/gc -run 'TestAdoptionBarrier_SerializesCreateWithSessionIdentifierLock|TestAdoptionBarrier' -count=1
• go test ./cmd/gc -run 'TestAdoptionBarrier|TestSyncSessionBeads_(CreatesNewBeads|DuplicatePoolSessionNameKeepsVisibleOwner|SetsManagedAlias|DoesNotRewriteOwnershipWhenAliasRepairFails|ValidatesPreStampedPoolAlias)' -count=1
• make test
• make check