When SelectServerAsync does not find a suitable server, it triggers an early heartbeat to accelerate detection of topology changes that might result in a suitable server becoming available.

Heartbeats should occur at two frequencies:

1. heartbeatFrequencyMS: a slower heartbeat when everything is operating normally
2. minHeartbeatFrequencyMS: a faster heartbeat whenever some task was unable to find a suitable server and is waiting to see if a topology change results in a suitable server becoming available

But even when there are tasks waiting for a suitable server to appear, heartbeats must not be sent more frequently than minHeartbeatFrequencyMS.