add custom move operations for ObjectStore::Transaction

[cbodley]

I wrote up a quick unit test to make sure that ObjectStore::Transaction's default move operations were doing what we expected them to, and it turns out that they weren't.

What I learned is that the compiler treats plain-old-data types specially when generating their default move operations. Because the builtin types don't have default initial values, it reverts to a normal copy and makes no effort to restore the source object to default values. See 'Trivial move constructor' at http://en.cppreference.com/w/cpp/language/move_constructor for more information.

I added a custom move constructor and move assignment operator to TransactionData that does restore the default values, and the unit tests pass as expected.

[yuyuyu101]

Not sure, do we need explicate std::move? otherwise it should be a copy construct?

[yuyuyu101]

ignore this. I'm looking in the wrong

[tchaikov]

lgtm

[cbodley]

I think there are some other fields of Transaction that should be reset on move as well, I'll look into that.

[cbodley]

I pushed an updated branch that adds the custom move operations to Transaction as well, to reset its members with builtin types (osr, use_tbl, coll_id, object_id) to their default values.

I considered moving those fields into TransactionData, so we could avoid overriding the move operations in Transaction. But I decided to leave them there, so that we have the move operations in place when people add more fields to Transaction in the future.