OperationShardVersion::get should return a pointer, not a mutable reference

OperationShardVersion is a copyable type, and OperationShardVersion::get(txn) returns a reference to "txn"'s OSV. If that value is captured into a local value variable instead of a local reference variable by omitting the &, the compiler will not fail, and this could lead to hard to find bugs.

OperationShardVersion::get should be changed to return a pointer, and its value should be captured into an OperationShardVersion* when it is captured at all, to avoid this risk.