Merge pull request #2009 in SNORT/snort3 from ~DAVMCPHE/snort3:lru_cache_for_hash...

Merge pull request #2009 in SNORT/snort3 from ~DAVMCPHE/snort3:lru_cache_for_hash to master

Squashed commit of the following:

commit 6f962204d41c0b1007992483f739db508e5d9c0d
Author: davis mcpherson <davmcphe@cisco.com>
Date:   Fri Jan 24 09:11:21 2020 -0500

    xhash/zhash: refactor duplicated code into a common base class, xhash/zhash will subclass this new base class

    utils: create memory allocation class based on sfmemcap functionality

    xhash: refactor XHash and HashFnc to eliminate c-style callbacks and simplify ctor options

    xhash: rename hashfcn.[cc|h] to hash_keys.[cc|h]

    zhash: refactor to use hash_lru_cache and hash_key_operations classes

    zhash: make zhash a subclass of xhash...eliminate duplicate code

    utils: add unit tests for MemCapAllocator class

    hash: add unit tests for new HashLruCache class - (PR review comments