Return 503 when web3signer gets overloaded

[siladu]

Currently if web3signer is overloaded, it can bring down the whole service.

It might be a better approach to detect saturation of resources and start returning 503s so the CL can respond accordingly.

Idea came from https://www.brendangregg.com/blog/2020-07-15/systems-performance-2nd-edition.html

Linear scalability of response time could occur if the application begins to return errors when resources are unavailable, instead of queueing work. For example, a web server may return 503 “Service Unavailable” instead of adding requests to a queue, so that those requests that are served can be performed with a consistent response time.