lease commands hook library should not use MultiThreadCriticalSections

Commands to add or update leases currently attempt to create MultiThreadCriticalSection(s) when they are unable to lock the target IP address. The original idea was that by entering CS all other work would stop and the update would be carried out. However, with HA+MT, the commands are being run by worker threads, which are not allowed to create critical sections. With the advent of #2043 (closed), attempting to create a CS will always fail, thus it would be better to simply fail when a lock cannot be obtained (or possibly retry), and fail with an error indicating a conflicting update. Thus the caller can decide what action to take.

There is little point in them attempting to create critical sections, thus the code should simply be removed.