Updated to xpt version 6

Changes in this version of xpt:
*) Now supporting developer fee (see main.cpp, line 183)
*) Mandatory ping/heartbeat (at least every 8 minutes)
*) Using compact format (4 bytes) instead of hash format (32 bytes) for
network and share target
I also rewrote the way xptClient connections are handled. The connection
object is now permanent and has a connected/disconnected state. Since
the same xptClient object may be accessed at all times this should
simplify synchronization between many threads.