Optimize dynamic mapping updates on master by processing latest one per index/node
Instead of processing all the bulk of update mappings we have per index/node, we can only update the last ordered one out of those (cause they are incremented on the node/index level). This will improve the processing time of an index that have large updates of mappings.
closes #4373
Add an `usage` key to the CPU section of OsStats.toXContent.

 This is just the sum of existing `sys` and `user`

Closes #4374
Query String caching could cause matched_filters not working

When searching with a query containing query_strings inside a bool query, the specified _name is randomly missing from the results due to caching.

Closes #4361.
Closes #4371.
- fixing update mapping tests for index operations so the number of request will be based on the index thread pool size
- added update mapping tests for bulk operations