Implement push support.

Pushing is achieved by inverting the roles on the established
connection, i.e. the client tells the server what data it should pull
from the client (PullMeRequest).

Role inversion is achieved by moving the server loop to the serverLoop
function of ByteStreamRPC, which can be called from both the Listen()
function (server-side) and the PullMeRequest() client-side function.

A donwside of this PullMe approach is that the replication policies
become part of the rpc, because the puller must follow the policy.

rpc/structs.go

@@ -12,7 +12,8 @@ const (
 	RTFilesystemVersionsRequest              = 0x11
 	RTInitialTransferRequest                 = 0x12
 	RTIncrementalTransferRequest             = 0x13
-	RTCloseRequest                           = 0x20
+	RTPullMeRequest                          = 0x20
+	RTCloseRequest                           = 0xf0
 )
 
 type RequestHeader struct {
@@ -62,6 +63,12 @@ const (
 	InitialReplPolicyAll        InitialReplPolicy = "all"
 )
 
+type PullMeRequest struct {
+	// if true, the other fields are undefined
+	Finished          bool
+	InitialReplPolicy InitialReplPolicy
+}
+
 type CloseRequest struct {
 	Goodbye string
 }