stop old neighbor thread if neighbor re-added (full credit to @Leibi133)

[alon-e]

@Leibi133 's comment formatted as a Pull Request
fixes #345

[alon-e]

what if neighbors are replaced with other neighbors,
this fix won't resolve that.

1. while removeNeighbor() closes the (Source & Sink) sockets, the thread keeps on running (same as while loop as touched here)

2. the HashMap keeps references to dead threads. so memory would not be freed by GC (?).

[bahamapascal]

Have made a test run over the last 24 hours and it's working great and smooth!

[alon-e]

merged but didn't close automatically.