updated to mention the latest extensions to the protocol

introduced also two new message types IDG (index-get) and IDR (index-response)
which can be used to obtain the index of a running node from the internal
communication channel
fix: shardcache_free_index() should be used to release all resources referenced by a storage index
added support for the IDG and IDR commands

still untested ... support in shardcache_client to allow testing will be added shortly