Allows calling UpdateLockPoints() with a (const) txiter. Note that this
was already possible for caller using mapTx.modify(txiter). The point
here is to not be accessing mapTx when doing so.
This is a refactor as long as no signed integer overflow appears. In
normal operation and absent bugs, signed integer overflow should never
happen in the touched code paths.
The main benefit of this refactor is to drop the file-wide ubsan
suppression unsigned-integer-overflow:txmempool.cpp.
For now, this only changes the internal private representation and the
publicly returned type remains uint64_t.
