New Memcached caching module

[antonpirker]

Create a new integration MemcachedIntegration.

Part of #2949

It should be similar as the RedisIntegration but as the first iteration only create cache spans as described here: #2965

The integration should be auto enabling and patch pymemcache.

(There is another lib that is still famous, but it is in maintenance mode and basically going to die: python-memcached We will not support this lib.)

Bonus:

• Load test the cache spans generation and publish results somewhere to be confident that this will not add to much overhead.