MB-8663 Add REST APIs for working with per replication settings.

API endpoint is /settings/replications/<replication id>

For GET it returns an object containing all the parameters that are
overridden for this replication.

POST updates the parameters.

Following is the list of currently supported per replication
parameters:

    maxConcurrentReps
    checkpointInterval
    docBatchSizeKb
    failureRestartInterval
    workerBatchSize
    connectionTimeout
    workerProcesses
    httpConnections
    retriesPerRequest
    optimisticReplicationThreshold
    xmemWorker
    enablePipelineOps
    localConflictResolution
    socketOptions

Change-Id: I3da280ee9981370bd997d0a55271eaebd92f8b11
Reviewed-on: http://review.couchbase.org/28181
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>