[#260,!120] Addressed more review comments

[#260,!120] Addressed more review comments

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