Fix row deduplication issues (#294)

## Description of Changes
Addresses
clockworklabs/SpacetimeDBPrivate#1585

There was an issue with row deduplication when a TransactionUpdate of
the form:

```
+ k1: v1
- k1: v1
- k1: v1
+ k1: v2
```
appeared, where `k1` is the primary key of a row and `v1`, `v2` are the
full values of that row. The old manner of processing updates would see
this as a no-op. I've updated the logic to handle this case correctly,
at the cost of an extra equality comparison per row in an update.
(Actually it's slightly cheaper than this, it skips the equality
comparison if there is only row with a particular primary key in an
update. This may help speed up processing of large updates e.g. chunk
data in BitCraft.)

## API

 - [ ] This is an API breaking change to the SDK

*If the API is breaking, please state below what will break*

## Requires SpacetimeDB PRs

## Testsuite

SpacetimeDB branch name: master

## Testing

- [x] Added a failing test, then fixed it

Author: kazimuth