Support executing multiple concurrent builds per Docker Machine

This adds the MaxConcurrentBuilds configuration field to the docker-machine executor.

Previously, each provisioned machine ran exactly one build at a time, leaving resources underutilized for heavy parallel pipelines. By tracking ActiveBuilds within the machineProvider, we now safely allow concurrent acquisition of machines that have spare capacity. We've also updated the provider to correctly signal availability back to the coordinator both when a machine drops to zero active builds and when it retains spare capacity after a release.