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

core/lib.rs

@@ -61,6 +61,7 @@ use std::{
 use storage::btree::{btree_init_page, BTreePageInner};
 #[cfg(feature = "fs")]
 use storage::database::DatabaseFile;
+pub use storage::pager::PagerCacheflushStatus;
 pub use storage::{
     buffer_pool::BufferPool,
     database::DatabaseStorage,
@@ -216,7 +217,7 @@ impl Database {
         let pager = Rc::new(Pager::finish_open(
             self.header.clone(),
             self.db_file.clone(),
-            Some(wal),
+            wal,
             self.io.clone(),
             Arc::new(RwLock::new(DumbLruPageCache::default())),
             buffer_pool,
@@ -503,7 +504,11 @@ impl Connection {
         self.pager.wal_frame_count()
     }
 
-    pub fn cacheflush(&self) -> Result<CheckpointStatus> {
+    /// Flush dirty pages to disk.
+    /// This will write the dirty pages to the WAL and then fsync the WAL.
+    /// If the WAL size is over the checkpoint threshold, it will checkpoint the WAL to
+    /// the database file and then fsync the database file.
+    pub fn cacheflush(&self) -> Result<PagerCacheflushStatus> {
         self.pager.cacheflush()
     }