Key pool feature for safer wallet backup

SVN rev 163 (ver 0.3.13.3) has the key pool feature.  Pre-generated new keys are aged in a queue before use, so that backups of wallet.dat hold keys you'll use in the future.

For now I made the default pool size 100.  It can be configured with -keypool=.  Be aware, it takes a little time to increase the pool size, so don't go crazy with it.  Disk space is about 1K per key.

I have not addressed the recovery side of this yet.  If you actually did restore an old wallet.dat, I think you may have to delete blk*.dat to rediscover your own transactions during the redownload.

I've only tested this moderately.  You might not want to use this for a website server until it's had some more testing.