silence Variant#in_stock? warning for correct usage

See comment here: https://github.com/spree/spree/commit/a7e6bc1#diff-f61dbf6358707ede7a8f24c85692baa5R147

It looks like it's still OK to call in_stock? without any arguments. If that's true then let's not print a deprecation notice when the method is called without arguments.
fix N+1 query performance issues on TaxRate.match method

Conflicts:
	core/app/models/spree/tax_rate.rb
Fix deprecation warning.
Add index to spree_orders for user_id & created_by_id

Addresses potential performance regressions discussed at #4582 in the last_incomplete_spree_order method arising after that method changed in c455e1a

Fixes #4584
Merge branch '2-1-stable' of github.com:spree/spree into 2-1-stable
Fix for issue #4234.