make concurrency of requests to external services configurable

Currently, the number of allowed concurrent requests is hard coded in places like the tm4 clock file.

Since processes like this are not the only client making requests to services like the tm4 api, it's possible the num of allowed concurrent requests is too high and the tm4 service barfs.

It is also possible the inverse is true and more requests can be made at the same time.

Adding an env var for the num of concurrent requests would allow for this flexibility.