Cache results of dbhash command for config collections.
When multiple mongos instances issue the dbhash command simultaneously, config database can be overloaded and affect performance.
It is present in versions of MongoDB prior to and including v2.4.6.
Cache dbhash commands on the server.
This is a performance improvement only.
Production release v2.4.7 contains the fix for this issue, and production release v2.6.0 will contain the fix as well.