chore(spanner): wait until queries have started before closing ReadContext

[olavloite]

Prevents a ReadContext from being closed before all queries have actually started. This extra check is needed now that queries in read-only transactions are started using the background executor when the async API is used.

Follow-up for #12715

[gemini-code-assist]

Code Review

This pull request updates the locking mechanism in AbstractReadContext.java by replacing the intrinsic object lock with a ReentrantLock and a Condition variable to manage pending asynchronous operations. It also introduces an AtomicInteger to track pending starts and ensures that the close() method waits for all pending operations to complete. I have reviewed the implementation and agree with the feedback provided regarding a potential resource leak: if the AsyncResultSetImpl constructor throws an exception, the pendingStarts counter will not be decremented, causing the close() method to hang. The suggested fix to wrap the constructor in a try-catch block is necessary to ensure the counter is correctly decremented in failure scenarios.