Support for simple interactive rollback like:
```sql
create table t (x);
insert into t values (1);
begin;
insert into t values (2);
rollback;
select * from t;
```
This PR also fixes some other issues I found while debugging:
* Checkpoint would never `clear_dirty` on pages in page cache.
* Auto commit for interactive transactions was not respected so any
`insert` after `begin` would flush frames regardless of `auto_commit`
state.
* `max_frame` on wal shared state was being updated after every
`append_frame` which was incorrect, as another transaction would be able
to use that new `max_frame` even tho the transaction could've rolled
back. Instead we update the private copy of `max_frame` and only update
it at the end.
Follow up for later are savepoints which require implementing a
subjournal to track savepoints and their modified pages.
Closes#1825
When `struct Database` is constructed, store `is_empty` as an
`Arc<AtomicBool>` - the value is true if:
1. DB size is zero
2. WAL has no frames
When `struct Pager` is constructed, this `Arc` is simply cloned.
When any connection runs a transaction it will first check `is_empty`,
and if the DB is empty, it will lock `init_lock` and then check `is_empty`
again, and if it's still true, it allocates page1 and stores `false` in
the `is_empty` `AtomicBool` and drops the lock.
---
Note that Limbo can currently have a zero DB and a WAL with frames,
as we have no special logic for folding page1 to the main DB file
during initialization.
Page 1 allocation currently happens on the first transaction (read or
write, due to having to support `select * from sqlite_schema` on an
empty DB; we should really check how SQLite actually does this.).
Previously we implemented update as a simple `Delete` + `Insert`
procedure which seemed okay for the moment but it wasn't. `Delete` can
trigger balance and a post balance `seek` which will leave cursor
pointing to an invalid page which `Insert` will try to insert to.
We solve this by removing `Delete` from the execution plan and rely on
`Insert` to properly overwrite the cell where the rowid is the same as
the one we are inserting.
Previously, with the `index_experimental` feature enabled, the query in
the added test would enter an infinite loop. This happened because
`label_grouping_agg_step` pointed to a constant argument that was moved
to the end of the program. As a result, the aggregation loop would jump
to the constant, then return to the start of the main loop, rewind the
index, and re-enter the aggregation loop—causing it to repeat
indefinitely.
