Redesigned the percolator engine to execute in a distribute manner.

With this design the percolate queries will be stored in a special `_percolator` type with its own mapping in the same index where the actual data is or in a different index (dedicated percolation index, which might require different sharding behavior compared to the index that holds actual data and being search on). This approach allows percolate requests to scale to the number of primary shards an index has been configured with and effectively distributes the percolate execution.

This commit doesn't add new percolate features other than scaling. The response remains similar, with exception that a header similar to the search api has been added to the percolate response.

Closes #3173