k-NN settings

The k-NN plugin adds several new cluster settings.

Cluster settings

Setting	Default	Description
`knn.algo_param.index_thread_qty`	1	The number of threads used for graph creation. Keeping this value low reduces the CPU impact of the k-NN plugin, but also reduces indexing performance.
`knn.cache.item.expiry.enabled`	false	Whether to remove graphs that have not been accessed for a certain duration from memory.
`knn.cache.item.expiry.minutes`	3h	If enabled, the idle time before removing a graph from memory.
`knn.circuit_breaker.unset.percentage`	75.0	The native memory usage threshold for the circuit breaker. Memory usage must be below this percentage of `knn.memory.circuit_breaker.limit` for `knn.circuit_breaker.triggered` to remain false.
`knn.circuit_breaker.triggered`	false	True when memory usage exceeds the `knn.circuit_breaker.unset.percentage` value.
`knn.memory.circuit_breaker.limit`	50%	The native memory limit for graphs. At the default value, if a machine has 100 GB of memory and the JVM uses 32 GB, the k-NN plugin uses 50% of the remaining 68 GB (34 GB). If memory usage exceeds this value, k-NN removes the least recently used graphs.
`knn.memory.circuit_breaker.enabled`	true	Whether to enable the k-NN memory circuit breaker.
`knn.plugin.enabled`	true	Enables or disables the k-NN plugin.