| This class monitors image production.
Currently it excersizes its control in two ways:
1) It determines whether image production is allowed or not
by checkingn the VM's memory state. Image production is
disabled when the state reaches a certain level.
By default, image production is disabled when the state
reaches VM.STATE_YELLOW.
2) It also determines how many image fetcher threads are allowed
to function concurrently. This number is by default 4.
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