…sing CacheInterface::get() (nicolas-grekas)

This PR was merged into the 4.2-dev branch.

Discussion
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[Cache] allow to skip saving the computed value when using CacheInterface::get()

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

While trying to play with using Messenger and Cache together, I spotted a few issues related to the new `CacheInterface::get()` method:
- there must be a way to skip saving the computed value in the pool. This is needed in my (still local WIP) use case - and is also needed for `LockRegistry`: processes that wait for the lock to be released should not write the value back when the lock is over. This is addressed by adding a 2nd `bool &$save` argument to the computing callback. When the callback sets this reference to true, the returned value should not be written to the pool.
- in order to better document the signature of this callback, a new `CallbackInterface` is added in the `Contracts\Cache` namespace. This will help autocompletion and implementations.
- `ContractsTrait` was writing twice the value to the pool - now fixed
- `LockRegistry` did not retry locking when a waiting process wasn't able to fetch the fresh value - now fixed.

Commits
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321d7f4 [Cache] allow to skip saving the computed value when using CacheInterface::get()