lib: make thread.c pthread-safe

lib: make thread.c pthread-safe

This change introduces synchronization mechanisms to thread.c in order
to allow safe concurrent use.

Thread.c should now be threadstafe with respect to:
* struct thread
* struct thread_master

Calls into thread.c for operations upon data of this type should not
require external synchronization.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>