SERVER-16067 Move LockRequestList to a separate header file
SERVER-16067 Simplify LockHead mode count tracking

Instead of using enumerations, use an increment/decrement calls in order
to avoid condition checking where it's not needed.

Inline all LockHead methods.
SERVER-16143 Remove RESOURCE_DOCUMENT and add a RESOURCE_METADATA
SERVER-16133 Use RESOURCE_METADATA X lock to set the multiKey index value

This avoids the need to do deadlock detection and as a nice side effect
would prevent the scary deadlock messages from showing in the log.

Also, is a matter of correctness, we should not be setting the _isMultiKey
field to TRUE until the transaction commits.