Improve Index cache lookup

Sometimes a workaround should look for uncommitted vertices within a transaction. To do this, we use the index cache. The first condition, which has the corresponding composite index, is used as the key to search the cache.

`StandardJanusGraphTx.java`
``` java
private PredicateCondition getEqualityCondition(Condition condition) {
...
for (final Condition child : condition.getChildren()) {
final PredicateCondition p = getEqualityCondition (child);
if (p! = null) return p;
}
...
}
```
Thus, if the selected index is not selective (i.e., it returns too many elements), then a cache search is not optimal.
I think that we need to iterate over all indexed keys and choose a more suitable one to search in the cache.

Start in StandardJanusGraphTx.java at getEqualityCondition and trace how its returned PredicateCondition is used for index-cache lookup. Review the indexed-key selection and cache behavior for transactions with uncommitted vertices; done means the lookup can consider all relevant indexed keys and avoid relying on a non-selective first condition.

Written by the indexing model from the issue text.