Reduce the number of blocks checked from 100 to 25.

[indolering]

Currently, nmc2couch checks 100 blocks and updates all of them by default. However, this is absolute overkill as the job is run every 10 minutes and the script verifies the entire chain every 24 hours.

The longest orphaned fork in Bitcoin history (as of this writing) was 25-30 blocks. The script should default to 25 and perform a full block-chain verification every 12 hours.