Implement concurrency limit for notary server

[yuroitaki]

Notarization is a resource consuming operation — to prevent being swamped by too many concurrent notarization requests, notary server should limit the number of notarizations that it will process at any point of time. Any notarization request beyond the limit will be put on hold, and served when there is available 'slot'.

One should look into the pros and cons of implementing this in the notary server application code itself or in a reverse proxy — factoring in UX and the fact that some notary server might run multiple versions in the same machine (e.g. PSE's). Also, we might need a queue system to decide which put-on-hold request to serve first.

Alternatively, horizontal scaling can be used to achieve the same objective.