Support perf-cost experiment for local backend

The local backend that deploys functions in Docker containers on the user machine has been developed for testing purposes. However, our users want to use this backend for invocations and experiments, and we should also support this. Currently, the local backend does not have the full functionality of a FaaS platform.

• Implement missing parts of the faas.System interface.
• Add HTTP triggers.
• Support cold invocations with container restart.
• Add limits on the number of parallel containers to avoid too much CPU and memory overhead.