fix(Session): Revert to non-blocking sessions (refs #4860)

Commit 3e47c82708155949b5059909cebd375eb53f19f0 introduced a regression
that blocked concurrent requests with same session id.

This reverts to the original non-blocking behaviour.
Merge pull request #1215 from eguaj/3.3-A4860

fix(Session): Revert to non-blocking sessions (closes #4860)