Instead of writing sequence chunks as soon as they're are created (as a
result of hitting chunk boundaries), only write them once they're
referenced - which only happens if those chunks are themselves chunked.
The effect of this is root chunks of collections/blobs aren't written
until they're committed, which makes the XML importer run twice as fast
on a month of MLB data - 60s instead of 120s, with --ldb-dump-stats
showing a PutCount of 21,272 instead of 342,254.
In the future it should be possible to avoid writing *any* chunks until
the root is committed, which will improve incremental update
performance, but that's a larger change (issue #710). This change fixes
issue #832.
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