sql/opt: add implicit SELECT FOR UPDATE support for UPSERT statements

[nvanbenschoten]

Address this TODO:

cockroach/pkg/sql/opt/exec/execbuilder/mutation.go

Lines 981 to 989 in 01a780b

	// tryApplyImplicitLockingToUpsertInput determines whether or not the builder
	// should apply a FOR UPDATE row-level locking mode to the initial row scan of
	// an UPSERT statement.
	//
	// TODO(nvanbenschoten): implement this method to match on appropriate Upsert
	// expression trees and apply a row-level locking mode.
	func (b *Builder) shouldApplyImplicitLockingToUpsertInput(ups *memo.UpsertExpr) bool {
	return false
	}

Specifically, UPSERT statements can occasionally hit a fast-path where they perform a blind PutRequest. However, for those that can't (e.g. writing a subset of columns or need to update a secondary index), an initial row scan must be used, just like with UPDATE statements. We should apply a FOR UPDATE row-level locking mode to the initial row scan of an UPSERT statement, in accordance with the enable_implicit_select_for_update session variable.