Merge pull request #2094 in SNORT/snort3 from ~SMINUT/snort3:expect_cache_min_prune...

Merge pull request #2094 in SNORT/snort3 from ~SMINUT/snort3:expect_cache_min_prune to master

Squashed commit of the following:

commit 9eb02b8dabe4bee0a03cbcbf0a991913f014d059
Author: Silviu Minut <sminut@cisco.com>
Date:   Fri Mar 20 22:30:54 2020 -0400

    flow: allow the ExpectCache to force prune, so that we can always make room when the cache is full.

    flow: change the ExpectCache prune logic to only remove a specified number of oldest entries, regardless of node expiration time.

    flow: do away altogether with the loop in ExpectCache::prune, just remove one, only when the cache is full.