Merge pull request #1483 in SNORT/snort3 from ~BBANTWAL/snort3:batching to master

Merge pull request #1483 in SNORT/snort3 from ~BBANTWAL/snort3:batching to master

Squashed commit of the following:

commit ecb607e1f70f760b545101a4dfa687f184aa2c36
Author: Jonathan McDowell <j.mcdowell@titan-ic.com>
Date:   Wed Jan 9 14:36:35 2019 +0000

    detection: Add search batching infrastructure

    Batch jobs for submission in fp_detect, allowing for a single
    submission of related buffers in the MPSE. Add a generic search_batch()
    which just calls search() for each MPSE instance, and a
    receive_responses() to provide initial infrastructure for asynchronous
    operation.