Fix concurrency crash in InternalDatabaseImpl

[benoitletondor]

I'm getting the following crash in production:

          Fatal Exception: java.util.ConcurrentModificationException:
       at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:760)
       at java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:782)
       at kotlin.collections.CollectionsKt___CollectionsKt.mapTo(CollectionsKt___Collections.kt:1628)
       at com.powersync.db.internal.InternalDatabaseImpl$1$2.emit(InternalDatabaseImpl.kt:75)
       at com.powersync.db.internal.InternalDatabaseImpl$1$2.emit(InternalDatabaseImpl.kt:74)
       at kotlinx.coroutines.flow.FlowKt__DelayKt$debounceInternal$1$3$1.invokeSuspend(Delay.kt:226)
       at kotlinx.coroutines.flow.FlowKt__DelayKt$debounceInternal$1$3$1.invoke(Delay.kt:11)
       at kotlinx.coroutines.flow.FlowKt__DelayKt$debounceInternal$1$3$1.invoke(Delay.kt:11)
       at kotlinx.coroutines.selects.SelectImplementation$ClauseData.invokeBlock(SelectImplementation.java:843)
       at kotlinx.coroutines.selects.SelectImplementation.complete(Select.kt:715)
       at kotlinx.coroutines.selects.SelectImplementation.doSelectSuspend(Select.kt:456)
       at kotlinx.coroutines.selects.SelectImplementation.access$doSelectSuspend(SelectImplementation.java:251)
       at kotlinx.coroutines.selects.SelectImplementation$doSelectSuspend$1.invokeSuspend(Select.kt:12)
       at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
       at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
       at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.java:586)
       at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:829)
       at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:717)
       at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:704)

This is because the "debounce + accumulate" implem isn't thread safe, and it's totally possible to edit accumulatedUpdates in collect while adding values to it in onEach.

Adding a Mutex should fix this. Note that you can probably monitor Kotlin/kotlinx.coroutines#1302 as it's really what's done here and some, probably more robust, implems are shared

[DominicGBauer]

Thanks a lot for this! Closing this PR as it is included in #120