Optimize DBSIZE command to not require lock on keypool

[JyotinderSingh]

DBSIZE store API currently takes a lock on the keypool to evaluate the number of keys in the database. Instead we can simply return the value stored in KeyspaceStat data structure which already counts the number of keys.

Additionally there is likely to be much lesser contention on this data structure as compared to the keypool, which will help with performance.

[raknay]

@JyotinderSingh Assign it to me.

[JyotinderSingh]

@JyotinderSingh Assign it to me.

Assigned.