Extending the chaincode handler and shim implementation to support
setting and retrieval of per-key metadata.

Change-Id: I95cd24c9dc34a3310e1c83a8e97ec638b14b0a8b
Signed-off-by: Matthias Neugschwandtner <eug@zurich.ibm.com>
Signed-off-by: Alessandro Sorniotti <ale.linux@sopit.net>