layout: Stop adding flows to the leaf set that are obviously never going
to be leaves.

60% improvement in flow tree construction time on the rainbow page.
auto merge of #1589 : pcwalton/servo/stop-removing-flows, r=larsbergstrom

to be leaves.

60% improvement in flow tree construction time on the rainbow page.

r? @larsbergstrom
util: Implement a Doug Lea-style concurrent hash table.

This will be used for leaf sets and interned strings.