Add EnableDataChannelBlockWrite to SettingEngine

The settingengine can enabled block write if
datachannel is detached, makes it working like
a normal net.Conn.

settingengine.go

@@ -103,6 +103,7 @@ type SettingEngine struct {
 	iceMaxBindingRequests                     *uint16
 	fireOnTrackBeforeFirstRTP                 bool
 	disableCloseByDTLS                        bool
+	dataChannelBlockWrite                     bool
 }
 
 // getReceiveMTU returns the configured MTU. If SettingEngine's MTU is configured to 0 it returns the default
@@ -121,6 +122,12 @@ func (e *SettingEngine) DetachDataChannels() {
 	e.detach.DataChannels = true
 }
 
+// EnableDataChannelBlockWrite allows data channels to block on write,
+// it only works if DetachDataChannels is enabled
+func (e *SettingEngine) EnableDataChannelBlockWrite(nonblockWrite bool) {
+	e.dataChannelBlockWrite = nonblockWrite
+}
+
 // SetSRTPProtectionProfiles allows the user to override the default SRTP Protection Profiles
 // The default srtp protection profiles are provided by the function `defaultSrtpProtectionProfiles`
 func (e *SettingEngine) SetSRTPProtectionProfiles(profiles ...dtls.SRTPProtectionProfile) {