Small cleanups to pager/wal/vdbe - mostly naming

- Instead of using a confusing CheckpointStatus for many different things,
  introduce the following statuses:
    * PagerCacheflushStatus - cacheflush can result in either:
      - the WAL being written to disk and fsynced
      - but also a checkpoint to the main BD file, and fsyncing the main DB file

      Reflect this in the type.
    * WalFsyncStatus - previously CheckpointStatus was also used for this, even
      though fsyncing the WAL doesn't checkpoint.
    * CheckpointStatus/CheckpointResult is now used only for actual checkpointing.

- Rename HaltState to CommitState (program.halt_state -> program.commit_state)
- Make WAL a non-optional property in Pager
  * This gets rid of a lot of if let Some(...) boilerplate
  * For ephemeral indexes, provide a DummyWAL implementation that does nothing.
- Rename program.halt() to program.commit_txn()
- Add some documentation comments to structs and functions

tests/integration/common.rs

@@ -1,4 +1,4 @@
-use limbo_core::{CheckpointStatus, Connection, Database, IO};
+use limbo_core::{Connection, Database, PagerCacheflushStatus, IO};
 use rand::{rng, RngCore};
 use rusqlite::params;
 use std::path::{Path, PathBuf};
@@ -86,10 +86,10 @@ impl TempDatabase {
 pub(crate) fn do_flush(conn: &Rc<Connection>, tmp_db: &TempDatabase) -> anyhow::Result<()> {
     loop {
         match conn.cacheflush()? {
-            CheckpointStatus::Done(_) => {
+            PagerCacheflushStatus::Done(_) => {
                 break;
             }
-            CheckpointStatus::IO => {
+            PagerCacheflushStatus::IO => {
                 tmp_db.io.run_once()?;
             }
         }