merge of #2583 zombie server processes

Found that not all rethinkdb server processes were being shut down because we were relying on __exit__ being called when it goes out of scope. But those handlers are not generally called when the process exits.
Tolerate vector clock conflicts in the blueprint

Previously, namespace_repo_t would crash if there was ever a vector clock conflict in the blueprint. Now it silently ignores the conflict; while this isn't guaranteed to preserve availability, at least the server will stay alive so that the user can fix the problem.
Add regression test for issue 2738.
Don't generate new blueprints unnecessarily.

Previously, we would generate a new blueprint for every table after a table_create() operation, which could lead to a lot of spurious vector clock conflicts. With this commit, we only generate a new blueprint for the newly created table.
Merge branch 'next' into daniel_geo