Merge lp://staging/~gmb/launchpad/not-nullify-bwa.result-bug-546774 into lp://staging/launchpad/db-devel

Make BugWatchActivity.result NOT NULL and add a default SYNC_SUCCESS BugWatchActivityStatus.

This branch adds a DB patch to make BugWatchActivity.result NOT NULL,
updates all existing BugWatchActivity's with a NULL result to have a
result of BugWatchActivityStatus.SYNC_SUCCESS, which is now our base
success status.

== database/schema/patch-2207-99-0.sql ==

 - This patch updates all existing BugWatchActivity records with a NULL
   result to have a result value of 9, which corresponds with
   BugWatchActivityStatus.SYNC_SUCCESS.
 - The patch also makes BugWatchActivity.result NOT NULL.

== lib/lp/bugs/doc/bug-watch-activity.txt ==

 - I've updated the tests to account for the fact that
   BugWatchActivity.result can't now be None.

== lib/lp/bugs/interfaces/bugwatch.py ==

 - I've added a SYNC_SUCCESS status to BugWatchActivityStatus.
 - I've added a list of BugWatchActivityStatuses considered successful
   which we can use in queries and so on. We plan to expand this later.

== lib/lp/bugs/model/bugwatch.py ==

 - I've updated addActivity() so that it now sets the result of the new
   BugWatchActivity entry to SYNC_SUCCESS if no other value is
   specified.

== lib/lp/bugs/scripts/checkwatches/scheduler.py ==

 - I've updated the scheduler query to query for activity without a
   success status rather than just querying for result is not null.

== lib/lp/bugs/tests/test_bugwatch.py ==

 - I've updated the test to ensure that the scheduler treats success
   results correctly (i.e. by ignoring them when it calculates when next
   to check the watch).