Add option to specify max threads based on num cpus

[aprudhomme]

Add an alternative method for specifying the maximum number of threads for a thread pool. Allows you to compute number of threads based on the number of cpus, with the formula: (numCPUs * multiplier) + offset bound by a min and max.

The max threads are still specified by the threadPoolConfiguration.max*Threads config keys. If the config value is a number, the number is used directly as max threads (status quo). If the config is a dict, field values are used to compute the max threads.

Config fields:

• min - minimum value for max threads, default: 1
• max - maximum value for max threads, default: INT_MAX
• multiplier - max threads formula multiplier, default: 1.0
• offset - max threads formula offset, default: 0

Example:

threadPoolConfiguration:
  maxSearchingThreads:
    min: 5
    multiplier: 1.5

[sarthakn7]

Can we use this ObjectMapper? https://github.com/Yelp/nrtsearch/blob/master/src/main/java/com/yelp/nrtsearch/server/utils/JsonUtils.java