Move the poll interval logic from the streaming source to corrdinator

[morozov]

Instead of pausing after not having seen any updates in a given database (source-level logic), the partitioned connector should pause after not having seen changes in any of its partitions. So this logic should be moved from SqlServerStreamingChangeEventSource to PartitionedChangeEventSourceCoordinator (#16).

Acceptance criteria:
When streaming changes, the connector should pause between iterations.

Previous logic:

while (true) {
    if (!hasChanges(databaseName)) {
        sleep();
    }
}

Required logic:

while (true) {
    hasChanges = false;

    for (databaseName : databaseNames) {
        if (hasChanges(db)) {
            hasChanges = true;
        }
    }

    if (!hasChanges) {
        sleep();
    }
}