[Java] the KeySelector API is unclear

The KeySelector API is slightly unclear, and it should be possible to make it a little more useable in practice.

In particular, imagine wanting to perform a range scan on the interval `[A,B)`. All `getRange()` calls in `ReadTransaction` are inclusive on start and exclusive on end, so it should be possible to write `getRange([A,B))` for all the possible `getRange` methods. For the `byte[]` and `Range` versions, this is pretty trivial. However, when using `KeySelector`, the API becomes confusing.

In order to do `getRange([A,B))` using `KeySelector`s, you have to issue

```
getRange(KeySelector.firstGreaterOrEqual(A),KeySelector.firstGreaterOrEqual(B))
```
which is confusing, because an instinctive (and incorrect) reading of the API would suggest that you would want
```
getRange(KeySelector.firstGreaterOrEqual(A),KeySelector.lastLessOrEqual(B))`
```
But doing that would be incorrect.

It would be nice (not essential, but nice) if there was a syntactically clean way of getting that particular scan correct. I propose adding a `KeySelector.exact()` functionality which would emulate the `firstGreaterOrEqual` in practice, but visually would look more obvious:
```
getRange(KeySelector.exact(A),KeySelector.exact(B))
```
is obviously a scan on `[A,B)` and will capture the intent of the caller without any special understanding by the caller themself.