perf(spanner): prune lock and begin event allocations on single-use snapshots

[olavloite]

Single-use snapshots (multi_use=False) are thread-confined and execute at most a single read or query operation. Previously, _SnapshotBase unconditionally instantiated both a Lock and an Event, and acquired locks during inline begin operations even when multi-use was not enabled.
This change optimizes single-use snapshots:

• Only allocate self._lock and self._transaction_begin_event when multi_use=True.
• For single-use snapshots, prevent sequential re-use in _wait_for_transaction_begin() via lightweight flag checks without lock synchronization.
• Skip lock acquisition in inline begin and precommit token updates when self._lock is None.
• Explicitly pass multi_use=True in Transaction.init to ensure transactions retain lock synchronization.
• Document concurrency semantics: single-use snapshots are thread-confined, while multi-use snapshots/transactions synchronize concurrent access.

[gemini-code-assist]

Code Review

This pull request optimizes single-use snapshots by avoiding the allocation of locks and events since they are thread-confined and do not run concurrently. The multi_use parameter is now passed to the base snapshot constructor, and lock-dependent methods have been updated to safely handle None values. Comprehensive unit tests have been added to verify this new behavior. I have no feedback to provide as the changes are correct and well-tested.