Workaround issue with greenthreads and lockfiles
Workaround issue with greenthreads and lockfiles

 * Adds a GreenLockFile that always works in greenthreads
 * Adds test to verify that regular Lockfile is broken
 * Adds test to verify that GreenLockfile works
 * Adds note about limitation of external locks
 * Adds test showing limitation of nested locks
 * Fixes bug 956313

Change-Id: I11cd1206611aa4862dadd2fcc077c4c2e0f798f6

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects