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][1] at for a description of why this is bad.

[1]: http://www.modrails.com/documentation/Users%20guide%20Apache.html#_smart_spawning_gotcha_1_unintentional_file_descriptor_sharing

Change-Id: I6019aaf3635c3f9f31076f6ac5d5d978978c718f
Reviewed-on: http://review.couchbase.org/25266
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>