REST API for 'max_parallel_indexers'

Based on the parameter 'max_parallel_indexers' , indexer are forked off.

The couchdb configuration system is at the moment not accessible to users by default, each setting needs an http rest handler in ns_server (and corresponding logic to propagate it to all other nodes). We want users to have the ability to increase the number of indexer via a REST API if needed depending on the machine they are using.

We need to update ns_config with this parameter to ensure that it is replicated to all cluster nodes.