ACCUMULO-3301 Fetch future tablet column from metadata.

We need to fetch both the future and current location for the tablets
in the table to be sure that the table has correctly transitioned
to the expected state. Without this, offline/online may incorrectly
return before the table has actually fully transitioned to its final state.

Lifted the code to create the scanner to its own method to easily
test it.
Revert "ACCUMULO-3292 Workaround which should make the test more stable."

This reverts commit b9d4c365b96558f1e46e9ef883e183b4ccba2884.

Unnecessary change as there actually was a bug in the coded which waits
for the offline (or online) to return that the system correctly moved
to the new state.
Start stubbing out the necessary hooks to track active assignments
Use the timer thread to scan over the active assignments and report ones that are still running after 15s

Try to grab the thread the task is executing in so we can use that
to grab a stacktrace when things get stuck.
Make a property to control how long an assignment takes before a warning is printed.
Introduce a property for controlling the warning threshold.
Whitespace