Merge pull request #1299 in SNORT/snort3 from stream_tcp_fin_handling to master

Merge pull request #1299 in SNORT/snort3 from stream_tcp_fin_handling to master

Squashed commit of the following:

commit 506ca045fd6219e2e8fcdc50735e9d8a0e36a20e
Author: davis mcpherson <davmcphe.cisco.com>
Date:   Wed Jun 27 10:45:15 2018 -0400

    stream_tcp: 86 r_nxt_ack as tracker state for next rx seq, use rcv_nxt instead

commit 45aa8684c57804309b298dcb4fe797274a09e987
Author: davis mcpherson <davmcphe.cisco.com>
Date:   Thu Jun 21 14:18:04 2018 -0400

    stream_tcp: back out fin handling changes for bug not relevant to snort3