Earlier detection of failed locking preconditions during commits

A CollabNet customer has requested earlier detection at commit time of the sorts
of lock-related precondition failures that will cause the whole commit to fail.
 Specifically, it would be nice if a commit doomed to fail because it attempts
to modify a locked resource without ownership of that lock would fail before the
costlier part of the commit process -- transmitting the modified content -- began.

The required program flow (announcing to the server the paths which are
scheduled for modification, providing any held tokens for those paths, and then
continuing to the actual modification phase) might be a better fit for Ev2
(Editor, v2 -- svn_editor.h).