taildrop: improve the functionality and reliability of put (#9762)

Changes made:
* Move all HTTP related functionality from taildrop to ipnlocal.
* Add two arguments to taildrop.Manager.PutFile to specify
  an opaque client ID and a resume offset (both unused for now).
* Cleanup the logic of taildrop.Manager.PutFile
  to be easier to follow.
* Implement file conflict handling where duplicate files are renamed
  (e.g., "IMG_1234.jpg" -> "IMG_1234 (2).jpg").
* Implement file de-duplication where "renaming" a partial file
  simply deletes it if it already exists with the same contents.
* Detect conflicting active puts where a second concurrent put
  results in an error.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>
Co-authored-by: Rhea Ghosh <rhea@tailscale.com>

ipn/ipnlocal/local.go

@@ -3550,12 +3550,13 @@ func (b *LocalBackend) initPeerAPIListener() {
 
 	ps := &peerAPIServer{
 		b: b,
-		taildrop: &taildrop.Handler{
+		taildrop: &taildrop.Manager{
 			Logf:             b.logf,
 			Clock:            b.clock,
 			Dir:              fileRoot,
 			DirectFileMode:   b.directFileRoot != "",
 			AvoidFinalRename: !b.directFileDoFinalRename,
+			SendFileNotify:   b.sendFileNotify,
 		},
 	}
 	if dm, ok := b.sys.DNSManager.GetOK(); ok {