[ML] Frequent Items: use a bitset for deduplication

[hendrikmuhs]

By using bitsets instead of lists of longs item sets can be faster de-duplicated. A bit is set according to the order of top items (by count).

Notes:

• the bitset might be useful for transactions and can speedup the lookup to find out if a candidate set matches the transaction
• bitsets reduce memory requirements(memory for remembering collected sets)

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[elasticsearchmachine]

Hi @hendrikmuhs, I've created a changelog YAML for you.

[benwtrent]

Those numbers look great! I like some of the refactors as well. Just some minor comments.