Key handle manipulation is not thread-safe in Mbed Crypto

[ionut-arm]

As mentioned here, Mbed Crypto is not yet thread safe when it comes to allocating or releasing key slots.
This came out of our own stress tests as well, with the service failing to handle requests or tripping over in a segmentation fault. The underlying problem was that all the threads processing requests were getting the same key slot allocated (number 32) and were modifying each other's keys.

This issue should be addressed by hiding the calls to create, open or close a key behind a mutex.