[DiskBBQ] avoid EsAcceptDocs bug by calling cost before building iterator

[iverase]

Nightly benchmarks are failing with the following error:

Caused by: java.lang.IllegalStateException: This operation only works with an unpositioned iterator, got current position = 2147483647
        Suppressed: java.lang.IllegalStateException: This operation only works with an unpositioned iterator, got current position = 2147483647

This happens whenever we call EsAcceptDocs#iterator and then EsAcceptDocs#bits and the underlaying structure is not a bitset.

More over if you try the combination EsAcceptDocs#bits and then EsAcceptDocs#iterator you get a different error:

 Exception in thread "main" java.lang.IllegalArgumentException: cost must be >= 0, got -1
        at org.apache.lucene.util.BitSetIterator.<init>(BitSetIterator.java:61)

Therefore in order to run those two methods you need to call EsAcceptDocs#cost. I am hacking this in this PR so we can get some nightly benchmarks data points.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)