Set difference when picking random ports

[dadgar]

The way that dynamic port assignment is done currently doesn't work well at all if significant ranges of reserved_ports are specified in nomad client config. For example:

client {
  reserved {
    reserved_ports = "23000-65535" 
  }
}

With this config (which leaves ports 20000-22999 available for assignment), running a job requiring several dynamic ports will fail more often than not, as a random pick in the range 20000-60000 will fall in a reserved range >90% of the time.

This change ensures that acceptable ports are always assigned, so long as a sufficient number of free ports exist in any non-reserved range between MinDynamicPort (20000) and MaxDynamicPort (60000).

Thanks to @bagelswitch for reporting.

@armon for review

[armon]

I would copy full bytes at a time not bits. You have access to the underlying byte array, and there is no need to add a conditional set.

[armon]

left some comments, LGTM

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