Introduce a stop downsample api

[martijnvg]

Downsampling can take a long time and unfortunately in erroneous situations, downsample operations may get stuck. We should be able to cancel a downsample operation for these cases.

Stopping a downsample operation would include the following steps:

• Mark a certain downsample operation as cancelled in downsample source index's IndexMetadata.
• Teach ILM/DSL not to attempt to downsample again for the current interval step that was stopped. ILM/DSL may decide that downsample the next interval step.
• Remove persistent task.
• Cancel node task.
• Remove the downsample target index.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)