Split up proof.dat file into smaller files

[kcalvinalvin]

trafficstars

For testnet3, the proof.dat file is around 13GBs. For mainnet, this is hundreds of gigabytes. It's not ideal having it as a one big file.

[adiabat]

Lets use the same formatting as the blk0000.dat and rev0000.dat files, so there can be a proof00000.dat, proof00001.dat, etc. They might go a little over the max file size for block.dats but should be OK.

[adiabat]

Also, please wait on fixing this (splitting up the proof file) until I merge (or abandon?) PR #206 , will have a bunch of annoying merge conflicts & should be easier after #206 is done.