[5332] Revamped lease_cmds hook lib to use hook callouts for command processing

[5332] Revamped lease_cmds hook lib to use hook callouts for command processing

The changes were patterned after what was done in subnets_cmds hook lib.

src/hooks/dhcp/lease_cmds/lease_cmds_callouts.cc
    New file which provides callout functions for each lease command
    as well as load() and unload() (formerly in load_unload.cc)

src/hooks/dhcp/lease_cmds/load_unload.cc
    File deleted

src/hooks/dhcp/lease_cmds/Makefile.am
    Added lease_cmds_callouts.cc
    Removed load_unload.cc

src/hooks/dhcp/lease_cmds/lease_cmds.h
src/hooks/dhcp/lease_cmds/lease_cmds.cc
    LeaseCmdsImpl - removed ctor, dtor, registerCommands, deregisterCommands,
    and changed handlers to non static

    LeaseCmds - added wrapper methods around LeaseCmdImpl handlers