Use LinkedHashMap for PacketExtensions in Packet

to keep the order in which the elements are added.

Some XEPs define a Schema using 'xs:sequence' (see for example XEP-60 §
17.1), so the order the PacketExtensions are added should be the same
they are transformed to a XML String.
Change Packet.getExtensionsXML()

Remove the synchronized, as getExtensions() is properly synchronized and will
return a copy.

Return XmlStringBuilder instead of CharSequence to take advantage of
fast XmlStringBuilder.append(XmlStringBuilder).

Mark the method as final, as it should not be overwritten.