Merge branch 'trac5324'

    Merge branch 'trac5314'

 - it's a copy 5272 code added that's already on master,
   but it seems easier to add it on a branch than go through
   the whole rebase process.

 - added missing comments for couple handlers
 - duplicated leaseX-update check removed, unit-test added
 - added missing parameters in lease_mgr_unitest.cc
 - added sanity check for state when adding and updating leases
 - not possibly anymore to sneak v4 addresses when v6 is expected
   (and vice versa)

doc/guide/logging.xml
    Updated logging section with more explanation of maxsize and maxver

src/lib/dhcpsrv/logging_info.cc
    LoggingInfo::toSpec()
        Now sets maxsize and maxver in created spec

src/lib/dhcpsrv/tests/logging_info_unittest.cc
    TEST_F(LoggingInfoTest, defaults)
        Added checks for default maxsize and maxver

src/lib/dhcpsrv/tests/logging_unittest.cc
    LoggingTest:
        Added support for log files

    TEST_F(LoggingTest, logRotate) - new test the ensures
    logs rotate when configured to do so

src/lib/testutils/dhcp_test_lib.sh.in
    Added code to remove log lock file.  When rotation is enabled,
    lock files are automatically enabled.

src/lib/dhcpsrv/tests/logging_unittest.cc
    TEST_F(LoggingTest, logRotate) - new test the verifies log rotation works.

    Pool6::toElement() will only include excluded-prefix if it isn't
    blank.

    Extracted logic for elementizing pools into toElement methods
    in Pool, Pool4, and Pool6 classes.

    Provided implementation for Subnet::toElement() which handles all
    common members, reworked Subnet<4/6>::toElement() to use this.