DelegatingSparkeyReader.

Used to simplify thread-local and reloadable sparkey readers.
Test reloadable sparkey reader.

Change-Id: Iaf3feacb3f4c85912712a627a7bf3efe0d2345b4
Merge pull request #11 from mbruggmann/marcbr/delegatingreader-squashed

Delegating sparkey reader.