Enable snapshot readConcern on mongos without atClusterTime

To enable running tests in the core transactions suite that use snapshot readConcern against unsharded collections on a single shard, mongos needs to allow snapshot readConcern for more commands than it currently does. These commands do not need atClusterTime, since snapshot readConcern will currently only be used against a single shard and will not need to establish a global timestamp.

The commands are:

• aggregate
• distinct
• find
• findAndModify
• insert
• update
• delete