Label-predicate: Add support for change owner votes

Change owner votes may have special meanings and different workflows
may be enforced for them:

o reject votes by change owner can denote Work In Progress (WIP) changes
o approval votes by change owner can mean multiple things, depending on
  project policies:
  oo review schould not be done on those changes and the change is
     only uploaded for verification purposes on different platform and
     architecture combinations
  oo auto_merge self approved changes when verification was successful
  oo enforcing "non-self approval policy", but not forbidding it to
     still be able to self approve changes in emergency cases.
     Given that this is exception from the policy, there should be an
     easy way to detect this policy violation with native gerrit
     predicate

Currently there is no built in way to detect change owner votes.
Having a built in way and not depending on external scripts to detect
change owner votes, would create addition value by allowing to combine
different predicates and use the result in gerrit itself, e.g.
dashboards can skip WIP changes.

This change extends label predicate semantic by adding additional
value for change owner votes: "label,owner". Schema version is bumped
to force reindexing. Label field is renamed to Label2 and query parser
is taught to route to the correct predicate depending on the live index
version.

With this change, change owner votes can be detected and used in
different workflows:

o skip WIP changes, rejected by change owner:
  is:open NOT label:Code-Review-2,owner
o skip non reviewable changes, approval by change owner:
  is:open NOT label:Code-Review+2,owner
o suggest changes for auto merge: approval by change owner + verify
  by the bot (assuming default label set: CRVW + VRFY):
  project:foo
  branch:master
  is:open
  is:mergeable
  label:Code-Review+2,owner
  label:Verified+1,buildbot
  NOT label:Code-Review-2
  NOT label:Verified-1
o detect changes, that violates "non-self approval policy":
  label:Code-Review+2,owner

Change-Id: I56ff6b1416f099dc627794c175aa4c2de3f2db9f