taildrop: add logic for resuming partial files (#9785)

We add the following API:
* type FileChecksums
* type Checksum
* func Manager.PartialFiles
* func Manager.HashPartialFile
* func ResumeReader

The Manager methods provide the ability to query for partial files
and retrieve a list of checksums for a given partial file.
The ResumeReader function is a helper that wraps an io.Reader
to discard content that is identical locally and remotely.
The FileChecksums type represents the checksums of a file
and is safe to JSON marshal and send over the wire.

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

@@ -3552,7 +3552,7 @@ func (b *LocalBackend) initPeerAPIListener() {
 		b: b,
 		taildrop: &taildrop.Manager{
 			Logf:             b.logf,
-			Clock:            b.clock,
+			Clock:            tstime.DefaultClock{b.clock},
 			Dir:              fileRoot,
 			DirectFileMode:   b.directFileRoot != "",
 			AvoidFinalRename: !b.directFileDoFinalRename,