Avoid creating unused streams

[jmacd]

Users have observed that an unused but configured otelarrow exporter will always create its configured num_streams, which consume a number of resources on the receiver side, leading to larger-than-necessary limits for intermediate load balancers.

This should be fixed, connected with #239. Let streams not start until there is load, let them terminate early when there is no load, and prioritize streams to prefer using fewer than the configured number of streams, if possible.