Add _cat/pending_tasks.

Closes #4251.
[DOCS] Doc'ed cluster pending tasks
Supply field descriptions for TimestampedTable.
Convert _cat/count to TimestampedTable.
Check for load avg before attempting to display.
Display placeholder for non-master-eligible nodes.
Completion postings format: Write hole character

Another issue found by randomized testing.
Refactored put mapping api to make use of the new recently introduced generic ack mechanism

Note: we were previously waiting for ack only from all nodes that contain shards for the indices that the mapping updatewas applied to. This change introduces a wait for ack from all nodes, consitent with other api as the ack is meant more on the cluster state itself, which is held by all nodes and needs to be updated on all nodes anyway.

Closes #4228
- Added docs for the value_count aggregation
- Fixed typos in the terms facets docs
- Fixed aggregation docs layout
- Added docs for shard_size in term aggregation