Update mapping on master in async manner
Today, when a new mapping is introduced, the mapping is rebuilt (refreshSource) on the thread that performs the indexing request. This can become heavier and heavier if new mappings keeps on being introduced, we can move this process to another thread that will be responsible to refresh the source and then send the update mapping to the master (note, this doesn't change the semantics of new mapping introduction, since they are async anyhow).
When doing so, the thread can also try and batch as much updates as possible, this is handy especially when multiple shards for the same index exists on the same node. An internal setting that can control the time to wait for batches is also added (defaults to 0).

Testing wise, a new support method on ElasticsearchIntegrationTest#waitForConcreteMappingsOnAll to allow to wait for the concrete manifestation of mappings on all relevant nodes is added. Some tests mistakenly rely on the fact that there are no more pending tasks to mean mappings have been updated, so if we see, timing related, failures down later (all tests pass), then those will need to be fixed to wither awaitBusy on the master for the new mapping, or in the rare case, wait for the concrete mapping on all the nodes using the new method.
closes #6648