Allow caching prepared statements

[simolus3]

This adds the preparedStatementCacheSize option, which is supported both for native (sqlite3_connection_pool) and web (sqlite3_web) databases. For web databases, the statement cache is implemented in Dart and managed by the web worker. For native databases, the cache is implemented in Rust and shared between isolates.

With a cache size of 10 statements, this provides a modest performance improvement compared to not caching any prepared statements:

Benchmark	Without cache	Cache enabled
Insert: JSON1	11.685 ms	11.575 ms
Insert: Direct	13.620 ms	12.845 ms
Insert: writeTransaction	9.0310 ms	7.7310 ms
Insert: executeBatch in isolate	13.842 ms	14.123 ms
Insert: direct write in isolate	60.972 ms	52.800 ms
Insert: writeTransaction no await	8.6580 ms	7.7120 ms

To be able to release this in a patch release without much risk, this does not enable statement caching by default.

[rkistner]

Adding a configurable cache seems like a good idea.

I think we should explicitly note the potential caveats of the cache in the docs. One that I've seen in the past is that an open statements can block dropping a table or index, such as in this example.

I also noticed the PR description mentions a default cache size of 10, but the code mentions 0 - I assume the latter is the correct default?

[simolus3]

One that I've seen in the past is that an open statements can block dropping a table or index, such as in this example.

It looks like that comment is outdated from powersync-ja/powersync-sqlite-core@c023c89 since we no longer have a prepared statement there, but it's still a good point in general. We call sqlite3_reset() on statements before putting them in the cache which I believe would have fixed the original issue in the core extension too, but it's still something worth pointing out.

I also noticed the PR description mentions a default cache size of 10, but the code mentions 0 - I assume the latter is the correct default?

Yes, I initially added a cache by default but considering that this is a patch release I think it's better to leave it disabled by default. I have updated the PR description.