The minimal change would be to disable shading of Guava in the module,
and rely on the transitive dependency from other libraries instead. But
since Guava's use is so localized, I think it's better to just not use
it instead, so I replaced that code and removed all traces of Guava from
the module's build.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #6555 from vanzin/SPARK-8015 and squashes the following commits:

c0ceea8 [Marcelo Vanzin] Add comments about dependency management.
c38228d [Marcelo Vanzin] Add guava dep in test scope.
b7a0349 [Marcelo Vanzin] Add libthrift exclusion.
6e0942d [Marcelo Vanzin] Add comment in pom.
2d79260 [Marcelo Vanzin] [SPARK-8015] [flume] Remove Guava dependency from flume-sink.