Fix https://github.com/NuGet/WebBackgrounder/issues/7 - unhandled Exceptions from executing query in RunInTransaction (from miscellaneous  failures such as SqlException) fail to restore _context to a valid state that allows further calls to RunInTransaction.
+Unit tests, add null checking tests, ensure EntityWorkItemRepository._context can never be assigned null, and simplify the Dispose() implementation to rely on transitivity for correctness.
Merge remote-tracking branch 'origin/Fix_#7'