[5332] Pass II on lease_cmds, refactored handlers to accept CalloutHandle

[5332] Pass II on lease_cmds, refactored handlers to accept CalloutHandle

Refactored the LeaseCmd(Impl) handlers to accept CalloutHandle rather
than command name and arguments. This does two things:

1. Passes the context into the handlers, should we ever want to
pass additional information into or out of the handlers

2. Greatly reduces code clutter