Make sure that a chaincode can invoke the new metadata-related getters and
setters only if the channel operates in 1.2 mode.

Change-Id: I15e77cd88a6e7ef0e802b02743a7e3cbdcc30ee3
Signed-off-by: Alessandro Sorniotti <ale.linux@sopit.net>
Signed-off-by: Matthias Neugschwandtner <eug@zurich.ibm.com>