Investigate BadgerDB database size

Seems like when using BadgerDB, database size keeps increasing.

I ran the benchmark with 3 nodes, until block 4000. And the size of the 3 databases are 1.3GB. Not sure if that's normal.

Use PR #353 to enable BadgerDB GC.

We also stopped using BadgerDB by default in PR #375 we should switch back to BadgerDB after figuring out what's up with the memory usage, which may be related to this issue (if not, we'll split the switching back to a separate issue)