The latter causes 'buffer type not supported' errors on OpenSUSE and
perhaps some other systems running MariaDB.

    Merges in #260,!120

	modified:   src/bin/dhcp4/dhcp4_messages.mes
    modified:   src/bin/dhcp4/tests/config_parser_unittest.cc
    modified:   src/bin/dhcp4/tests/get_config_unittest.cc
    modified:   src/bin/dhcp6/tests/get_config_unittest.cc

    Removed RCV_ prefix from WatchedThread::WatchtType members
    Scrubbed WatchedThread source for lingering packet/receiver comments
    Adjusted test wait times

doc/guide/congestion-handling.xml
    Added subsections to Congestion Handling chapter

src/lib/dhcp/iface_mgr.*
    Removed Receiver class, replaced with new class lib::util::thread::WatchedThread
    Renamed IfaceMgr::receiver_ to dhcp_receiver
    Renamed add_fd to addFDtoSet

src/lib/dhcp/packet_queue_mgr.h
    PacketQueueMgr::unregisterFactory() - destroys queue instance
    if it matches type being unregistered

New files:
src/lib/util/threads/
    watched_thread.h
    watched_thread.cc

src/lib/util/threads/tests
    watched_thread_unittest.cc