Add CfgConsistency defaults to application level parsing

src/lib/dhcpsrv/cfg_consistency.h
    Changed constructor default mode to LEASE_CHECK_NONE

src/lib/dhcpsrv/lease_file_loader.h
    LeaseFileLoader::load() - modified to create and use a
    sanity_checker only if checking is enabled.  This avoids
    senselessly making the same decisions for every lease loaded.

src/lib/dhcpsrv/parsers/simple_parser4.*
src/lib/dhcpsrv/parsers/simple_parser6.*
    Added sanity checks defaults so they can be properly set at
    the application level.

src/lib/dhcpsrv/sanity_checker.h
    SanityChecker::leaseCheckingEnabled() - new static function
    to test if sanity checking for leases is enabled

src/lib/dhcpsrv/tests/sanity_checks_unittest.cc
src/lib/dhcpsrv/tests/srv_config_unittest.cc
    updated tests for new constructor default value