fix(gatsby-plugin-mdx): generate mdx once when multiple mdx fields requested

[vladar]

Description

This PR is a performance improvement for one specific usage scenario of the MDX plugin. It addresses inefficiency in the plugin when multiple GraphQL fields are requested that require MDX processing. An example query that is affected:

{
    mdx {
      mdxAST
      body
    }
}

In this case, the plugin starts two MDX processing tasks concurrently as shown on this chart:

This PR accounts for such a scenario and only processes MDX once. For MDX-heavy sites using this pattern, e.g. https://github.com/newrelic/docs-website

This translates to the following difference in query running time:

Before this PR:

success run page queries - 2776.543s - 8324/8324 3.00/s

After this PR:

success run page queries - 1919.154s - 8324/8324 4.34/s

(image processing also affects those numbers, on a machine with more CPU cores or in Gatsby Cloud where image processing is additionally optimized it can yield even bigger difference)

[LekoArts]

Published in gatsby-plugin-mdx@2.10.1