api: Current:currents needs to be static to avoid deadly embrace.

Discovered by findbugs:
org.ovirt.engine.api.common.invocation.Current$1 needs
to be _static_ to avoid a deadly embrace
with org.ovirt.engine.api.common.invocation.Current.currentsFirst.

By definition: ThreadLocal instances are typically private static
fields in classes that wish to associate state with a thread.

Change-Id: I86516044e758aa526f7bab570d41567b35176029
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>