ACCUMULO-4502 Added volative to some class members to mitigate issues…

[ivakegg]

… when one thread seeks the iterator, and another thread is reading it.

[ivakegg]

After an exhaustive search through this code, the only reason I can come up with is an issue with a member reference in the underlying HeapIterator. The scenario would be as follows:

thread 1: calls hasTop() which is invoked on then in memory data source
thread 2: calls switchSource which sets up the new datasource and calls seek which results in topIdx being set
thread1: calls next() which gets a value of null for topIdx

So if topIdx is marked as volatile then this should not happen.