…e or all, closes elastic#2041.

Allow to configure store throttling (only applied on file system based storage), which allows to control the maximum bytes per sec written to the file system. It can be configured to only apply while merging, or on all output operations. The setting can eb set on the node level (in which case the throttling is done across all shards allocated on the node), or index level, in which case it only applied to that index.

The node level settings are indices.store.throttle.type to set the type, with values of none, merge and all (defaults to none). And, also, indices.store.throttle.max_bytes_per_sec (defaults to 0), which can be set to something like 1mb.

The index level settings is index.store.throttle.type for the type, with values of node, none, merge, and all. Defaults to node which will use the "shared" throttling on the node level. And, index.store.throttle.max_bytes_per_sec (defaults to 0).