Optimization: use recoveryUnit to avoid read on updates

[benmccann]

From https://jira.mongodb.org/browse/SERVER-15793:

It would seem that the storage engine's recoveryUnit could just hold on to the pointer or the length itself when dataFor is called if that is an optimization that helps the storage engine.

[igorcanadi]

Tnx @benmccann. Yeah, I think it's worth playing around with this idea some more. Basically in RecoveryUnit we can detect write transaction and if it's write transaction just cache the size of all keys read (we only need the size of the value, not the actual value). We're currently busy with getting this baby up n running in production and 'update' performance is not on the top of our todo list. But it's definitely a low-hanging fruit we shall tackle once we start to optimize.