Allow SetMatrix to be used as a value type with a ready-to-use zero
value. SetMatrix values are already non-copyable by virtue of having a
mutex field so there is no harm in allowing non-pointer values to be
used as local variables or struct fields. Any attempts to pass around
by-value copies, e.g. as function arguments, will be flagged by go vet.
Signed-off-by: Cory Snider <csnider@mirantis.com>
9e3a6ccf69