Prevent multiple simultaneous calls to the servers endpoint

[lorenzo-cavazzi]

Avoid overlapping queries to the /servers endpoint by implementing a very basic semaphore logic.

The first commit comes from #498, merge after that.

[lorenzo-cavazzi]

Rebased and ready to test. Preview available here: https://lorenzotest.dev.renku.ch

To test it, go to any page where notebooks are visualized (e.g. https://lorenzotest.dev.renku.ch/notebooks), open the network tab and try to slow down the connection so that a single response from the /servers API will take more than 3 seconds to come (e.g. from the chrome debugger, select a custom device with network throttling very bad).

It's actually easier to test by setting the POLLING_INTERVAL variable down to a very low value (e.g. 300 instead of 3000), but this requires to checkout the branch locally and running it through telepresence.

[vfried]

I tested and it works well, the code looks good also.

Well done!