fix overflow problem

[sriharivishnu]

When switching from a long running task that has not terminated, there are often errors when the output from the last command are carried over to a new fragment. Example : ServicesFragment (treehouses remote allservices) -> StatusFragment

#734 uses delimiters to find when a command has started and ended, and keeps a queue of commands in progress.

This PR solves the issue by killing all running commands by sending a special command to the bluetooth server when switching fragments (Specifically, called in the OnDestroyView of BaseFragment).

Bluetooth branch required:

• Please test this PR with this branch: https://github.com/treehouses/control/tree/multithread

treehouses upgrade bluetooth multithread
treehouses bluetooth restart

should work; otherwise, manually copy server.py into /usr/local/bin/bluetooth-server.py