Makebootstrap

Hi,

i had to export bootstrap file from some favorite altcoins. There is already an export script, but that is very slow, because blocks are queried from rpc. So i added some funcions to the coin's codebase, so the bootstrap export is accelerated by this. I want to share my code.

The are two commit in this push: one made the export by running the daemon/wallet. the second allow made the bootstrap in a running daemon/wallet.

The command line parameter required a filename or filepath. The bootstrap data is exported to this file. There are another two helper parameter, this allow exporting a part of the blockchain: the first blocks (minheight) and the last blocks (maxheight).

The rpc command do the same: require a filename/filepath parameter, and the optional first/last blockheight.
If no first/last are given, the whole blockchain is exported (1..bestblockheight).

If you like this feature, feel free to give some notes.

Note: the rpc exporting sould made by a separete thread, but i dont know how to modify the [TraceThread](https://github.com/bitcoin/bitcoin/blob/master/src/util.h#L200) function to pass parameter(s).