Optimized advanceSynapsesDevice kernel function invoking

1. Changed the name of struct InverseMap to SynapseIndexMap, and added
activeSynapseIndex to the struct.
2. the activeSynapseIndex contains synapse indexes of all active
synapses.
3. now advanceSynapsesDevice kernel function is invoked by the number
of active synapses, and the function refer the activeSynapseIndex to
get the synapse index to process.