[3601] Memfile_LeaseMgr now runs LFC automatically if lease files need upgrading

[3601] Memfile_LeaseMgr now runs LFC automatically if lease files need upgrading

Memfile_LeaseMgr loads the lease files during instantiation. If it detects
that any of the files loaded are out of date, it will invoke LFC directly,
whether or not LFC is enabled.

src/lib/dhcpsrv/dhcpsrv_messages.mes
    added two new log messages:
        DHCPRSV_MEMFILE_UPGRADING_LEASE_FILES
        DHCPSRV_MEMFILE_NEEDS_UPGRADING

src/lib/dhcpsrv/lease_file_loader.h
    LeaseFileLoader::load() - emits DHCPSRV_MEMFILE_NEEDS_UPGRADING log
    when an out of date file is detected

src/lib/dhcpsrv/memfile_lease_mgr.cc
    LFCSetup::setup() - added run_once_now parameter, which causes the
    method to invoke the LFC callback immediately, regardless of the value
    of LFC interval.
    Memfile_LeaseMgr::Memfile_LeaseMgr() - added logic to track if
    files loaded need upgrading and pass that into lfcSetup()

    Memfile_LeaseMgr::loadLeasesFromFiles() - change to return a boolean
    true if any of the files loaded need upgrading.

    Memfile_LeaseMgr::lfcSetup() - added upgrade_needed parameter, which
    is passed through to LFCSetup::setup() as "run_once_now"