Initial Store & Forward Implementation (#2917)

This adds store & forward relays into dendrite for p2p.
A few things have changed:
- new relay api serves new http endpoints for s&f federation
- updated outbound federation queueing which will attempt to forward
using s&f if appropriate
- database entries to track s&f relays for other nodes

test/db.go

@@ -101,7 +101,6 @@ func currentUser() string {
 // Returns the connection string to use and a close function which must be called when the test finishes.
 // Calling this function twice will return the same database, which will have data from previous tests
 // unless close() is called.
-// TODO: namespace for concurrent package tests
 func PrepareDBConnectionString(t *testing.T, dbType DBType) (connStr string, close func()) {
 	if dbType == DBTypeSQLite {
 		// this will be made in the t.TempDir, which is unique per test