Reset Couchbase#thread_storage when a Kernel#fork is detected. Adding to...

... avoid file descriptor sharing between a process and its sub process. See section 13.3 at http://www.modrails.com/documentation/Users%20guide%20Apache.html for a description of why this is bad.

Change-Id: I6019aaf3635c3f9f31076f6ac5d5d978978c718f

Dupe of http://review.couchbase.org/#/c/25266/