Cancel pending requests when didChange & et all appear.

[mkaput]

Then any of the textDocument/did* requests come, we need to cancel all operations that are holding snapshots of the AnalysisDatabase in order to unlock mutations to it.

This task is spin off #5355

[mkaput]

We have researched this with @Draggu. Conclusions:

Cancellation is well-defined in the following Salsa RFC: https://murek.dev/salsa-book-0.16/rfcs/RFC0007-Opinionated-Cancelation.html

When you do a write to the salsa database, that write will block until any queries running in background threads have completed. You really want those queries to complete quickly, though, because they are now operating on stale data and their results are therefore not meaningful. To expedite the process, salsa will cancel those queries. That means that the queries will panic as soon as they try to execute another salsa query.

Running cancellation is not dependent on any work on tower-lsp→lsp-server migration or any significant changes to LS state. Salsa should automatically initiate cancellation of running tasks, our job is just to catch cancelled task execution and properly report this fact to the client.

The problem is, this RFC is not implemented in Salsa 0.16 😭 We will have to upgrade to Salsa 0.17; probably the fork maintained by Rust Analyzer: https://crates.io/crates/ra_ap_salsa