Reduce File IO for BucketList writes

[SirTyson]

Currently, we do lots of unnecessary file IO when writing a new batch to the BucketList. Specifically, we create a fresh bucket from a vector of entries, write it to disk, and open the file to index the bucket. Then we open the new bucket file and the current level 0 curr bucket file and kick of a very low priority background thread. This background thread does a merge via file IO, then again reindexes the merge output. During all this, we are blocking the main thread, waiting on this low priority background work.

Instead of launching a background thread, we should do all this work on the main thread. We should also only index buckets once, and since we have all the entries in memory already anyway, we should avoid file IO for level 0 merges.