add optimization which coalesces multiple updates on a relvar into one conditional scan/update

The following example updates can be coalesced into one conditional update:

```
update x where y=3 (z:=5); update x where y=4 (z:=6)
```

This will require updating the internal update expression to support multiple conditional branches. This optimization can coalesce n scans into 1 scan whenever the same relvar is targeted, however, the order of the execution is critical- the updates cannot happen in parallel.