Consider distinct array elements before formulating array index keys

Consider the case of a distinct covering array index where array has large number of entries e.g. 100K but only 100 are distinct. In such cases, indexer will first try to create all possible 100K entries and then do a distinct. If it is non-distinct, then count is also computed during distinct.

Entries are formulated in splitSecondaryArrayKey
https://github.com/couchbase/indexing/blob/master/secondary/indexer/array.go#L36

and later the distinct/count is done:
https://github.com/couchbase/indexing/blob/master/secondary/indexer/array.go#L147

But this can lead to large memory allocation request here:
https://github.com/couchbase/indexing/blob/master/secondary/indexer/array.go#L125

It is better to first do the distinct/count to avoid peak memory allocation.