SQLite: Cache prepared statements behind sql.exec().
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…ltiple statements. `prepare()` does not allow you to pass code that contains multiple statements (separated by semicolons). Extending it to allow this isn't straightforward, because later statements in the block might not be possible to parse if earlier statements haven't yet executed. For example, the first statement might create a table, and the next statement might insert into it. If you try to parse each statement before executing any of them, SQLite will throw an error on the second statement, because the table it's trying to insert into doesn't exist. To support a multi-statement prepare, we must change the contract such that the whole thing is parsed lazily, on the first actual execution. This is needed in order to support caching of prepared statements behind `exec()`, since `exec()` supports passing multiple statements at once.
This API is deprecated and won't be made public, so let's simplify the implementation to minimize maintenance burden. This commit also changes `exec()` to take `JsString` instead of `kj::String` because caching will be keyed by the JS string handle, so that's what prepared statements need to hold onto.
…chedColumnNames Will need to be here so it can be kept in the statement cache.
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sql.exec()currently parses the query every time it is invoked. This change makes it so that we retain some number of prepared statements in a cache and reuse them, keyed by the query text. Since query strings are likely commonly internalized strings, cache lookups should be O(1) most of the time, as internalized strings compare equal by identity without the need to compare content.