Performance Degradation in Long-Running Spanner Emulator Sessions

I've been utilizing the Spanner Emulator for testing purposes within my environment, paired with PHPUnit.
The feature set has been performing exceptionally well.
However, I've noticed a significant slowdown during prolonged testing sessions. Specifically, the slowdown appears to coincide with the creation of new connections.

For reference, please see the related issue #72 and the reproduction project at https://github.com/kt81/spanner-emu-tester.

Below is an example of the performance degradation observed:

```
...
$ make run
13.6603ms
17.372299ms
31.821499ms
46.6992ms
55.764599ms
70.637199ms
83.588399ms
92.9404ms
107.0446ms
116.8905ms
128.591101ms
146.4209ms
153.661801ms
166.057102ms
...
```

This pattern suggests a gradual decrease in performance, particularly with an increase in the number of new connections over time. Any insights or suggestions on how to mitigate this issue would be greatly appreciated.