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.
Don't generate new blueprints unnecessarily.

Previously, we would generate a new blueprint for every table after every db_create(), db_drop(), table_create(), or table_drop() operation. This lead to a lot of spurious vector clock conflicts. With this commit, we only generate a new blueprint after table_create() and only for the newly created table.
Add regression test for issue 2738.
Add some includes that aren't strictly necessary but should be there anyway.
Merge branch 'v1.13.x' of github.com:rethinkdb/rethinkdb into v1.13.x