[dss] sort cell ids in queries

[Shastick]

Sorts cell identifiers (usually stored in the cells column of various entities) in CRDB queries.

This is expected to reduce contention in two ways:

• similar queries will be guaranteed to access data in a similar order, helping with distributed locking
• sorted arrays may be easier to compare with the && operator

All queries that contain a condition of the form cells && <some_int64_array> depend on sql.CellUnionToCellIds() or sql.CellUnionToCellIdsWithValidation(), so this PR simply added slices.Sort() step to both of these methods.

First step of #814

[barroco]

Could you please provide evidence of the expected improvement ?