fix(fuse): a write opened as an upload lands takes the landed base (B170)

An upload that landed let go of its blob before the tree had its node. A
write open in between took the base it sampled at its start, the revision
the upload replaced, found none of its bytes cached, and queued an
incomplete revision; the next write open failed with EIO.

The drain keeps the landing node aside until the tree has it, and a write
open with no queued revision takes its base from that node, or from the
tree as it is when the handle goes in. The fake Drive can hold the drain
in that window (`hold_next_landing`).

docs/CHANGELOG.md

@@ -68,6 +68,8 @@ by its local id, in the queue, the stored tree and pins alike.
   to upload them to the top of your Drive instead (B168).
 - **Stopping the daemon during a folder sync no longer waits for the sync.** The uploads in
   progress finish, and the rest waits for the next start (B169).
+- **Writing to a file again just as its upload finishes no longer fails.** A write right after
+  it failed with "Input/output error" (B170).
 - **A file renamed just as its upload finished keeps its new name.** The mount no longer showed
   the old name, and a second rename no longer failed with "No such file or directory" (B152).
 - **A change that waits for a name to be freed no longer waits seconds after it is.** A rename or