[btree_builder] Fix reference counts of unshifted child values

The ref counts now are handled in a straightforward method:
The pre-built subtrees serve as snapshots of potentially shared nodes.
They hold an initial ref count on the pre-built nodes and their children.
The temporary ref counts are later dropped at the end of device building.

This way fixes the ref counts of unshifted nodes, thus the 'reserve'
operation introduced by commit 6d16c58 is reverted.

src/write_batcher.rs

@@ -18,9 +18,11 @@ pub struct WriteBatcher {
     batch_size: usize,
     queue: Vec<Block>,
 
-    // The reserved range covers all the blocks allocated or reserved by this
-    // WriteBatcher, and the blocks already occupied. No blocks in this range
-    // are expected to be freed, hence a single range is used for the representation.
+    // The reserved range keeps track of all the blocks allocated.
+    // An allocated block won't be reused even though it was freed.
+    // In other words, the WriteBatcher performs allocation in
+    // transactional fashion, that simplifies block allocationas
+    // as well as tracking.
     reserved: std::ops::Range<u64>,
 }
 
@@ -83,22 +85,6 @@ impl WriteBatcher {
         Ok(Block::zeroed(b))
     }
 
-    pub fn reserve(&mut self) -> Result<Block> {
-        let mut sm = self.sm.lock().unwrap();
-        let b = find_free(sm.deref_mut(), &self.reserved)?;
-        self.reserved.end = b + 1;
-
-        Ok(Block::new(b))
-    }
-
-    pub fn reserve_zeroed(&mut self) -> Result<Block> {
-        let mut sm = self.sm.lock().unwrap();
-        let b = find_free(sm.deref_mut(), &self.reserved)?;
-        self.reserved.end = b + 1;
-
-        Ok(Block::zeroed(b))
-    }
-
     pub fn get_reserved_range(&self) -> std::ops::Range<u64> {
         std::ops::Range {
             start: self.reserved.start,