MR !1128 introduced this code but on second reading I realized the
double-copy from this MR can be remove by using `try_emplace` and also by actually giving the `TxInfo` type a real constructor.

This commit is just a minor code quality commit to reduce the creation of a temporary.  In most cases the optimizer will elide this temporary anyway, but for debug builds it may not, and also it's of higher code quality to just avoid creating it.

ninja all check-all