Inner ring nodes do not collect signatures for the cheque now.
Instead they invoke "InnerRingUpdate" method and smart-contract
checks if method was called from inner ring node. Then it
accepts cheque if there were 2/3n+1 invokes.
With neo:morph environment, there will be no direct communication
between inner ring nodes and storage nodes. neo:morph smart-contracts
will identify inner ring nodes by their signatures.
Inner ring nodes check their presence in inner ring list during
startup. "IsInnerRing" method allows to check presence efficiently
by calling this method.
