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

setup/config/config_federationapi.go

@@ -18,6 +18,12 @@ type FederationAPI struct {
 	// The default value is 16 if not specified, which is circa 18 hours.
 	FederationMaxRetries uint32 `yaml:"send_max_retries"`
 
+	// P2P Feature: How many consecutive failures that we should tolerate when
+	// sending federation requests to a specific server until we should assume they
+	// are offline. If we assume they are offline then we will attempt to send
+	// messages to their relay server if we know of one that is appropriate.
+	P2PFederationRetriesUntilAssumedOffline uint32 `yaml:"p2p_retries_until_assumed_offline"`
+
 	// FederationDisableTLSValidation disables the validation of X.509 TLS certs
 	// on remote federation endpoints. This is not recommended in production!
 	DisableTLSValidation bool `yaml:"disable_tls_validation"`
@@ -43,6 +49,7 @@ func (c *FederationAPI) Defaults(opts DefaultOpts) {
 		c.Database.Defaults(10)
 	}
 	c.FederationMaxRetries = 16
+	c.P2PFederationRetriesUntilAssumedOffline = 2
 	c.DisableTLSValidation = false
 	c.DisableHTTPKeepalives = false
 	if opts.Generate {