makeClusterPSOCK() to not attempt to load 'parallelly' on the worker

[HenrikBengtsson]

The parallelly package is loaded on the worker because makeClusterPSOCK() calls parallelly:::session_info() in order to (i) test that the created worker workers, and to (ii) gather session information for more informative error messages in case of failures later.

It should be possible to avoid loading of parallelly by stripping session_info() of the 'parallelly' namespace environment