Enhance and cleanup DatabaseUpgradeChecker

Using a hierarchy of database version rather than a flat
list of them. Adding a new schema upgrade path was really
cumbersome and error-prone, because we needed to maintain
a flat map of versions and their corresponding list of
upgrade paths (`DbUpgrade`). Instead we're using a logical
hierarchy structure of versions:

```
DatabaseVersionHierarchy.builder()
    .next("4.0.0"   , new Upgrade40to41())
    .next("4.0.1"   , new Upgrade40to41())
    .next("4.0.2"   , new Upgrade40to41())
    .next("4.1.0"   , new Upgrade410to420())
    .next("4.1.1"   , new Upgrade410to420())
    .next("4.2.0"   , new Upgrade420to421())
    ...
    .next("4.2.1"   , new Upgrade421to430())
    .next("4.9.3.0" , new Upgrade4930to41000())
    .next("4.10.0.0", new Upgrade41000to41100())
    .next("4.11.0.0", new Upgrade41100to41110())
    .build();
```

With this change, when we need to add a new version upgrade
path, we only need to add it in correct place in the hierarchy
rather than add that in dozens of places in `_upgradeMap`.

Author: khos2ow