Replace LMDB with Bolt

This is the minimal set of changes needed to replace LMDB with Bolt as the storage engine:
* the directory layout is similar except the factors/ and shard/<shardId>/ directories are now files, each being a standalone Bolt db; the tables/ directory is unchanged
* each shard db has top level buckets for tables containing second level buckets for objects
* events are stored inside the object buckets keyed by their timestamps, the value being the same msgpacked event body as before
* factors db is flat with single top level bucket containing all factor id/values
* the query engine employs a copy of boltdb c_cursor (due to cgo importing issues) to provide the luajit with read access to the bolt dbs

@benbjohnson 

/cc @snormore, @charlescng