Dominic/indexing perf enhancements

Some indexing performance and safety enhancements.

Do not use abort rejection handler for DiscoverWriter threadpool

    Do not use abort rejection handler for DiscoverWriter threadpool. The
    abort rejection handler throws an exception and causes work to be lost
    if no worker is available for a job.

    We do NOT want to throw away work, so use the default rejection handler
    which effectively ensures all work is done, and effectively throttles
    ingestion.


Configure DiscoveryWriter threadpool size by itself

    The DiscoveryWriter threadpool does not need to be very big. Create
    configuration new config options specific to DiscoveryWriter TP size for
    maximum control when tuning.


Do not re-index existing metrics in ES

    Use the `CREATE` optype to use the "put-if-absent" functionality in
    Elasticsearch. This will cut down on extra and unnecessary indexing
    operations and IO in elasticsearch.