Merge pull request #2699 in SNORT/snort3 from ~MIALTIZE/snort3:version to master

Merge pull request #2699 in SNORT/snort3 from ~MIALTIZE/snort3:version to master

Squashed commit of the following:

commit fde481c81ff3499cd9b5cf8f18557a4801378021
Author: Michael Altizer <mialtize@cisco.com>
Date:   Wed Jan 13 12:57:23 2021 -0500

    build: Generate and tag 3.1.0

commit d8ba67eba1dac5e7e6ef19b02d252c4f1f6985f4
Author: Michael Altizer <mialtize@cisco.com>
Date:   Wed Jan 13 12:57:23 2021 -0500

    build: Add support for version sublevel and build via CMake

    The sublevel version in CMakeLists.txt acts as a fourth digit for the
    version.  It is expected to be 0 in the master branch in all but the
    rarest cases.

    The VERSION_BUILD CMake variable can be defined and used by an external
    build system for artifact tracking.  If a build number is not manually
    defined, all mention of build numbers will be stripped from Snort's
    output.  To set VERSION_BUILD from configure_cmake.sh, the
    SNORT_BUILD_NUMBER variable is used from the command line options.

    Note: A build number of 0 will be ignored and treated as though it was
    not set.

commit 8dff1244a18d88b2f2f3da7241f335d7f97159ed
Author: Michael Altizer <mialtize@cisco.com>
Date:   Wed Jan 13 12:57:23 2021 -0500

    stream_tcp: Remove obsolete flush_data_ready() function