[PULL] add -keypoolmin, and avoid creating new key on every TX

[xf2_org]

URL: https://github.com/bitcoin/bitcoin/pull/294

Create -keypoolmin, and only top up keypool if size falls below that

Avoid creating a new key -every- time you use a reserve key. Instead, create them in bursts.

Currently, default is to generate 25 keys (filling keypool up to 100), if keypool size falls below 75.

[Luke-Jr]

Can we increase the amount to generate too? I think it's perfectly reasonable to keep a buffer of 1000+ keys. Maybe a warning to backup when more are created, too?