avro

AVRO-1781: Use Guava for a thread-safe weak identity cache.

This adds avro-guava-dependencies to minimize just Guava classes that
are shaded without minimizing classes from other dependencies. This
filtered artifact is then shaded and relocated in the avro module to
avoid leaking Guava classes to downstream consumers.

git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1724473 13f79535-47bb-0310-9956-ffa450edef68