Cache the parser availability of the roundtrip feature

As otherwise there would be multiple log statements (which are of level
fine but nevertheless).

PacketParserUtils: XmlPullParser does not support XML_ROUNDTRIP org.xmlpull.v1.XmlPullParserException: unsupported feature: http://xmlpull.org/v1/doc/features.html#xml-roundtrip (position:START_DOCUMENT null@1:1)
PacketParserUtils: 	at org.kxml2.io.KXmlParser.setFeature(KXmlParser.java:2091)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:150)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:172)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection.openStream(XMPPTCPConnection.java:963)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketWriter.access$2600(XMPPTCPConnection.java:1224)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketWriter$1.run(XMPPTCPConnection.java:1263)
PacketParserUtils: XmlPullParser does not support XML_ROUNDTRIP org.xmlpull.v1.XmlPullParserException: unsupported feature: http://xmlpull.org/v1/doc/features.html#xml-roundtrip (position:START_DOCUMENT null@1:1)
PacketParserUtils: 	at org.kxml2.io.KXmlParser.setFeature(KXmlParser.java:2091)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:150)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:172)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection.openStream(XMPPTCPConnection.java:963)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader.access$200(XMPPTCPConnection.java:970)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader$1.run(XMPPTCPConnection.java:989)
PacketParserUtils: XmlPullParser does not support XML_ROUNDTRIP org.xmlpull.v1.XmlPullParserException: unsupported feature: http://xmlpull.org/v1/doc/features.html#xml-roundtrip (position:START_DOCUMENT null@1:1)
PacketParserUtils: 	at org.kxml2.io.KXmlParser.setFeature(KXmlParser.java:2091)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:150)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:172)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection.openStream(XMPPTCPConnection.java:963)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader.access$200(XMPPTCPConnection.java:970)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader$1.run(XMPPTCPConnection.java:989)
PacketParserUtils: XmlPullParser does not support XML_ROUNDTRIP org.xmlpull.v1.XmlPullParserException: unsupported feature: http://xmlpull.org/v1/doc/features.html#xml-roundtrip (position:START_DOCUMENT null@1:1)
PacketParserUtils: 	at org.kxml2.io.KXmlParser.setFeature(KXmlParser.java:2091)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:150)
PacketParserUtils: 	at org.jivesoftware.smack.util.PacketParserUtils.newXmppParser(PacketParserUtils.java:172)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection.openStream(XMPPTCPConnection.java:963)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader.access$200(XMPPTCPConnection.java:970)
PacketParserUtils: 	at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader$1.run(XMPPTCPConnection.java:989)
Fix initialization order issue with DEBUG_ENABLED

If a user enabled Smack debug via the property 'smack.debugEnabled', a
ConnectionConfiguration could be created where debuggerEnabled is
'false', because Smack is not yet initialized.

Also make sure that if the property is not set, it won't overwrite
DEBUG_ENABLED = true.

Thanks to William Murphy for providing a detailed issue description and
supposing a fix.