[5556a] Addressed review comments

[5556a] Addressed review comments

src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.cc
    LeaseMgrDbLostCallbackTest::testNoCallbackOnOpenFailure() {
    - removed before hand checks for socket descriptor, grab
    the most recently opend descriptor after connecting to the db
    and assume its the SQL client's

    - test return value of close() rather then errno

src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.h
    LeaseMgrDbLostCallbackTest
    - constructor and destructor clear
    DatabaseConnection::db_lost_callback

src/lib/dhcpsrv/tests/host_mgr_unittest.cc
    HostMgrDbLostCallbackTest
    - Setup and Teardown methods clear DatabaseConnection::db_lost_callback

    HostMgrDbLostCallbackTest::testDbLostCallback()
    - removed before hand checks for socket descriptor, grab
    the most recently opend descriptor after connecting to the db
    and assume its the SQL client's

    - test return value of close() rather then errno

src/lib/dhcpsrv/tests/test_utils.cc
    int findLastSocketFd()
    - iterate over all descriptors rather than stopping at first invalid