[0.13.1] Support multiple fedpegscript/signblockscript depending on blockheight

If for whatever reason the federation keys need to be changed but we don't want to start a new chain, the user should be able to set command-line flags that do so.

One idea is in addition to `-fedpegscript=<hex>` allow `-fedpegscript=<height>:<hex>`. Legacy parameter sets the hex value from genesis block onwards until the next highest height-hex pair.