Implement contact-locking

Note: Initial discussions were held via email with followups on IRC on Monday (16-Jul-12) and Wednesday (18-Jul-12). An extra discussion document was included on Wed ( http://pastebin.com/BieCSzXa ). A blog post explored the issue further. Another IRC conversation (13-Aug-12) settled on optimistic locking.

Some key design decisions:

1. Locks are optimistic
2. Locks apply coarsely to all core contact tables – civicrm_contact, civicrm_address, civicrm_phone, civicrm_email, civicrm_website, civicrm_im, and single-value custom-data
3. The key used to check revisions will be a "modified" timestamp on civicrm_contact
4. Locks are only checked for the "Edit Contact" and "View Contact - Inline Edit" screens. Other screens will not read the "modified" timestamp or display warnings/errors about it; however, other screens will update the "modified" timestamp.