Model transaction rate fetched from config file

As discussed with @jcrubino on hangouts

Currently the transaction rate is not variable. Need to fetch the config file, parse it and generate blocks as per the given height vs transaction rate.

Need a way to generate blocks according to the transaction rate so looking at enabling `SetGenerate` when number of txns are fulfilled and disabling it as soon as new block is mined.
I'd use GetBalance here, since it doesn't have any side effects.