Limit number of user threads

Presently jclouds does not limit the number of user threads. While de-asyncification could address this in the long-term, in the short-term we should protect our users with some sane default. I suggest 20 or 50, the former mirroring the number of IO threads, and the latter sourced from my extensive testing of most blobstore providers. Note that Atmos providers have issues with large numbers of concurrent requests and errors from JCLOUDS-137 will occur. I work around these in my application by setting 10 IO and 10 user threads for Atmos Online and 8 and 8 for Synaptic Atmos.