doesn't allow it

It can be useful to only synchronize zones that use a certain source. For
example, in a situation where some zones use a dynamic source and others don't,
you probably want to synchronize those with a dynamic source regularly, and
only synchronize the others when a change is made.

Although we only synchronize the zones that use a given source, we still want
to synchronize all sources to avoid deleting records that would live in another
source of the zone.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

* spelling: ancillary
* spelling: antarctica
* spelling: australia
* spelling: authentication
* spelling: continental
* spelling: constructor
* spelling: conversion
* spelling: creation
* spelling: doesn't
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* spelling: efficiently
* spelling: equivalent
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* spelling: supports
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* spelling: invocation
* spelling: itself
* spelling: leftovers
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* spelling: pointing

This change hit a line length limitation, so I'm wrapping it and
adding a period which appears to match local style...

* spelling: quicker
* spelling: response
* spelling: requested
* spelling: redirect
* spelling: traffic
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* spelling: uploaded
* spelling: useful
* spelling: separately
* spelling: zone