fix persistedQuery memory leak in SSR scenarios

[phryneas]

This fixes a potential memory leak in SSR scenarios when many persistedLink instances were created over time.

Before this change, hashesByQuery was a global WeakMap with a structure of

{
  [query1]: {
    forLink0:
      "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad",
    forLink1:
      "88d4266fd4e6338d13b845fcf289579d209c897823b9217da3e161936f031589",
  },
  [query2]: {
    forLink0:
      "36bbe50ed96841d10443bcb670d6554f0a34b761be67ec9c4a8ad2c0c44ca42c",
    forLink1:
      "bef57ec7f53a6d40beb640a780a639c83bc29ac8a9816f1fc6c5c6dcd93c4721",
  },
};

These hashes were not removed from memory when the link was discarded - and in a SSR scenario, where an ApolloClient was recreated for each request, and the links also were recreated (which is often a good idea), those hashes could accumulate a fair share of memory.

This PR makes hashesByQuery a per-link scoped variable in the form of

{
  [query1]: "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad",
  [query2]: "36bbe50ed96841d10443bcb670d6554f0a34b761be67ec9c4a8ad2c0c44ca42c",
};

If the link is discarded of, the whole set is discarded of as well, and memory is freed up again.

As a bonus, we save a few bytes of bundle size.

We might also want to add to the documentation that if the generateHash function is expected to always return the same output for the same input, it is generally safe to just create one link globally instead of one per SSR run.
This way, there is no need to calculate the hashes again for every SSR run, saving CPU time.

This is an advice we should not generally give for all links, but in this case it should be safe.

Checklist:

• If this PR contains changes to the library itself (not necessary for e.g. docs updates), please include a changeset (see CONTRIBUTING.md)
• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[changeset-bot]

🦋 Changeset detected

Latest commit: 55dc3d2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@apollo/client	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[phryneas]

/release:pr

[github-actions]

A new release has been made for this PR. You can install it with npm i @apollo/client@0.0.0-pr-10805-20230425110644.

[jerelmiller]

Looks great! Thanks so much making sure our SSR continues to function nicely!