SERVER-14006 Experimental document-level locking for in-place updates

This commit enables experimental (and untested) document-level locking for
in-place updates, which do not modify indexed values.

It should absolutely not be used on production systems and is for
demonstration purposes only.

It runs under server parameter called useExperimentalDocLocking. In order
to enable it, run:

./mongod <other parameters> --setParameter useExperimentalDocLocking=true