Add optional argument to getsnapshot to add last_active_height #249

Currently it's easy to get a list of all taddrs with funds but it's not clear which were more recently active. We can add a 2nd optional boolean argument to getsnapshot to include this data. Example call would be something like hush-cli getsnapshot -1 true where -1 tells getsnapshot to include all data, not just the top N entries. Or hush-cli getsnapshot 1000 true to show the last active height for the top 1000 taddrs.

With this data it would be easy to see which addresses are actively mining and which addresses have not had any "real" transactions since being airdropped funds from our old mainnet. Addresses with no real transactions since the airdrop are likely to be lost/unrecoverable funds.