remove checks in AbstractBinaryStoreTest that were beyond the BinaryStore contract.
move start/shutdown methods from AbstractBinaryStore up to the BinaryStore interface
relax JcrRepository's binaryStore to support any BinaryStore impl
add BinaryStore#hasKey, for checking if a key is in the store
Create a CompositeBinaryStore that:

- initializes any number of named BinaryStore instances
- appear as a vanilla BinaryStore to existing code
- allow consumers to offer a "StrategyHint" that MAY be used to
  determine what named BinaryStore to persist content to
- on retrieval, look for the BinaryKey in all named BinaryStores