SCHEMA-117. SchemaHashCache now uses a WeakIdentityHashMap.

Adds unit-test.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>
POM-4. Add kiji-annotations library to dependency management of the root pom.

This commit modifies kiji-schema to comply with POM-4, which modifies the
Kiji root pom to include the kiji-annotations library in its dependency
management section.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>
SCHEMA-122. Remove duplicate header.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>