fix: cursor leak when cur == nil and aux or conds is not empty

[stuartcarnie]

• symptom would be locked .tsm files resulting in an accumulation of .tsm.tmp files after a compaction

Scenarios

• aggregate queries that included fields in the where condition, where the aggregate field does not exist for all shards. example select count(*) where some_field = 0 in which case conds cursors will leak
• selectors that include additional fields and the field in the selector does not exist across all shards, like select first(foo), v0 from m0 limit 10, in which case aux cursors will leak

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

[jwilder]

Could you do something like:

type CursorsAt []cursorAt
func (a CursorsAt) Close() {
    for _, c := range a {
        c.close()
    }
}

And then CursorsAt(itr.aux).Close() or maybe change the type of aux and conds.curs to CursorsAt so you can just call itr.aux.Close()?

[stuartcarnie]

I should have gone with that first, as it did cross my mind. I'll update it

[stuartcarnie]

I'll keep it unexported, given cursorAt is also