Implement blocking strategy for writing to busy DOM nodes

[brson]

Our current implementation uses copy-on-write, but there is some skepticicsm about how performant this will be. Simply blocking and waiting for layout to finish could be faster than copying nodes. A blocking strategy would also eliminate a pointer indirection on each node. There is work that content can do while waiting (GC).

It should be possible to create a single interface that supports both strategies. For the sake of measurement this seems like a worthwhile thing to do.

[brson]

Recent discussions between @pcwalton and @bzbarsky may indicate that this isn't desirable after all.

[jdm]

We currently block, except when we don't. But should.