Enable rest api for XDCR parameters

Enable rest apis for tunable parameters for XDCR. Given the importance of XDCR and these tunables, we need REST endpoints. Suggestion is to add to /internalsettings

[

{xdcr_max_concurrent_reps, 32}

, % num of concurrent reps per bucket - Validate the user input to range min: 8 - max: 256 default: 32

{xdcr_checkpoint_interval, 1800}

, % in secs - - Validate the user input to range min: 60 - max: 14400 default: 1800

{xdcr_worker_batch_size,500}

, % num of mutations - Validate the user input to range min: 500 - max: 10000 - default 500

{xdcr_doc_batch_size_kb, 2048}

, % in kilobytes - Validate the user input to range min: 10 - max: 100000 - default 2048

{xdcr_failure_restart_interval, 30}

, % in secs - Validate the user input to range min: 1 - 300 default: 30

Current workaround is using diag/eval as
curl -X POST http://Administrator:asdasd@127.0.0.1:9000/diag/eval -d 'rpc:call(node(), ns_config, set, [xdcr_checkpoint_interval, 2000]).'