MSP-11605
`Rex::ThreadFactory.provider` needs to be set in
`Msf::Framework#initialize`, but setting it directly to
`Msf::Framework#threads` eliminates the laziness of
`Msf::Framework#threads`. In order keep `framework.threads` lazy,
`framework` is wrapped in a
`Metasploit::Framework::ThreadFactoryProvider`, which responds to
`spawn`, which is needed by `Rex::ThreadFactory`, by calling
`framework.threads.spawn`, which lazily initialized `framework.threads`
when the first thread needs to be spawned.
Luke Imhoff