Set default per page size to 200 when bulk indexing

[roborourke]

ElasticPress now supports setting the default bulk indexing page size via filter. Lets drop it to 200 to make indexing more reliable.

This also needs to account for the number of shards per node, and should be a number divisible by the number of shards so that ES can more effectively parallelise document ingestion.

Filter is ep_index_default_per_page

Acceptance criteria:

• Default to 200 documents per page in bulk indexing
• Round to nearest lower number divisible by number of shards for the index, e.g. 5 shards = 200, 3 shards = 180

[roborourke]

Related to #307