Use expvar counters instead of spamming logs

It's still interesting to know how many times we hit each case.
Skip indexing empty documents

No need to index only the id and nothing in it.
Tweak handling of $unset and $set

If we get both operations, we pick the $unset way of handling it:
get the full object from mongodb.

Added tests that requires mongodb, optionally enabled using flags.
Less byte slice making

Start with a buffer that can at least fit our set max size.
Avoids unnecessary growing of initial buffer.