mongodump/mongoexport should issue its own lock command

Mongodump and mongoexport have two forms of operation. They either connect to a running mongod instance or they operate directly on a mongo data directory. What I learned recently from Kristinia's blog (http://www.snailinaturtleneck.com/blog/2010/07/26/mongodb-backups-corn-on-the-cob-in-10-minutes/) was that it does not lock the database before dumping for ensure consistency.

There should be a switch to opt into a locking the database, I propose l/-lockDatabases.This will simplify backup scripts.