remove ability to try to lock the same resource twice

I searched for "wait_on_self" and only found it set in its own test.
With wait_on_self defaulting to false if something tried to lock the
same resource twice it would unlock its current lock and then acquire a
new lock.  This should almost certainly be a fatal error because it
probably means something else forgot to unlock its lock or if this is a
means of "sharing" a lock it means something else could acquire the lock
out from under this process.

CHANGELOG: It is no longer possible to lock the same resource twice (in
the same process).
use a default creation_time (epoch)

To avoid uninitialized value warnings use a default value for
creation_time (epoch).  Otherwise assumptions about creation_time being
defined, e.g. trying to sort allocations by creation_time, may produce
uninitialized value warnings.

Test that creation_time is set to ensure this didn't break the
creation_time accessor.

CHANGELOG: Genome::Disk::Allocation's creation_time now returns a
default value, the epoch, instead of undef.