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Parsoid: Add the variant proxy

The proxy allows directing requests to either variant. It loads both
variants' modules internally and uses their operations to complete
requests. It is designed in such a way so as to allow an easy transition
between fully using JS to fully using PHP with no config changes. When
first introduced, its defaults emulate the JS-only scenario. Once the
switch is fully achieved, then simply changing `sys/parsoid.js` for
`sys/parsoid-php.js` in `projects/sys/default.wmf.yaml` with no config
change results in having a fully-functional Parsoid/PHP module. The
proxy can. thus, function properly with only one of variant modules
loaded and configured.

In order to support the transition period, the proxy has three modes of
operation: single, mirror and split. In single mode, only one variant is
used, defined by the `default_variant` configuration value, defaulting
to `js`. This allows us to start using the proxy with no config changes.
In the final stages of the transition (before we remove the proxy), it
can be changed to `php` to only use the PHP variant. The mirror mode is
used to asynchronously mirror traffic to the PHP variant. Requests are
issued to both variants, but only the JS one is returned. The amount of
traffic to be mirrored can be tuned with the `percentage` configuration
parameter. The imporant caveat here is that only requests for
`/page/{format}` end points are mirrored - we cannot do so reliably for
transforms since they rely on stashed content, which is likely not to be
available for the PHP variant. Furthermore, when the proxy is configured
in mirror mode, dependency update events are emitted only for the JS
variant, so as to avoid duplicates. Finally, the split mode is used to
split the traffic between the two variants based on the request domain.
If one of the patterns given in the `pattern` configuration parameter is
matched, then the variant not defined in `default_variant` is used,
otherwise the default on... (continued)

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