This commit fixes issues found from using a more complex `testing/quick`
implementation of the `WriteIndex()` test. The newer test inserts
multiple sets of random data that's confined to a smaller random space
so there's more chance of overlapping data.
The fixes were primarily around inserting old data or inserting the same
timestamp multiple times for a single write. The block splitting was not
working correctly before and the sorting and deduping was not handled
correctly.
Ben Johnson
Jason Wilder