Improve query planning in StorageReadObjects

[ftkg]

This aims to optimize queries for cases where all requested storage objects are for the same user/key/collection:

1. For any requested column, if all requested values are the same then it becomes a simple WHERE $column = $value condition;
2. If among all requested columns only one has multiple distinct values requested, it becomes a WHERE $column = ANY($values[]) condition;
3. Otherwise, if we have two or three columns with multiple distinct values requested, these columns become part of a JOIN ROWS FROM () virtual table clause.

[sesposito]

The changes look good but because this a critical code path I'd suggest we add some tests to cover the different built queries.