Asynchronous offset commit requests should go directly to group_coordinator

[k32]

Currently brod_group_consumer:commit/4 function uses brod_group_consumer gen server as a proxy. However, this process can be blocked by CbModule:handle_message callback, and commit requests will linger in its message queue until it processes kafka message sets. This leads to undesirable delay until async offsets are committed. Also commit requests can be lost on restart. Proposal: this call should go directly to brod_group_coordinator.