[INTERNAL] make sure the thread pool is stopped on component dispose

The thread pool was not stopped on dispose and so the cleanup depends on
finalization which is bad.

Small refactor: changed visibility from protected to private for
variables and methods.

Change-Id: I78832b3cc859c7e8bd94e1915253da53f2277790
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1153
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>