[INTERNAL] cleaned up SharedDocumentProvider

Made fields private, overworked class Javadoc, removed useless
method overrides and added instance of checks. In addition this class
should now be thread safe.


Change-Id: I6ee770aaa211629ffd0905e3e95e0de17b42a5b3
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1850
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>