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.
log.Fatal(f) terminates the program, so either remove the dead code here (each return statement) and change the function to not return an error, or just return the error without the Fatal.