Add Semaphore utility to help implement connection pools

[simolus3]

We currently have two manual connection pool implementations in this repository: @powersync/node and @powersync/op-sqlite. Both implementations share a similar architecture:

1. We use a Mutex to guard access to the write connection.
2. For each read connection, we store a busy flag and have a queue of readers backed by an array.

For VFS implementations that allow concurrent access on the web, we'll soon have a third connection pool implementation. Adding the third copy-paste is always a good signal to refactor, so this PR adds utilities for implementing connection pools to @powersync/common.

The Semaphore class wraps an array of inner elements and can give out leases wrapping a single or multiple such elements. These are all acquired atomically, and requests on semaphores can be aborted individually. This is pretty much the same implementation we already had for pools, except that:

1. We use a slightly more efficient queue implementation by avoiding the O(n) Array.shift() when removing elements.
2. We also use a queue for non-busy connections which allows obtaining a lease without iterating through all of them.
3. By using our Semaphore implementation, we get to abort readers (which both @powersync/node and @powersync/op-sqlite didn't really support before).
4. close() and refreshSchema() now use the respective connection locks as well, avoiding potential concurrency issues.

Hopefully this is the last refactoring item for concurrent reads on the web :D

[changeset-bot]

🦋 Changeset detected

Latest commit: e6d0c2d

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

This PR includes changesets to release 8 packages

Name	Type
@powersync/node	Patch
@powersync/op-sqlite	Patch
@powersync/common	Minor
@powersync/adapter-sql-js	Patch
@powersync/react-native	Patch
@powersync/tanstack-react-query	Patch
@powersync/web	Patch
@powersync/diagnostics-app	Patch

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