Update bdb.

[nodech]

This is based on and closes #16

Main goal:

• Fix segfaults that can happen when db is closed and any method is called. Now they instead return errors (throw exception for sync/return err via callback)

Other issues:

• Find potential memory leaks on close and if those can accumulate. (sanitizer w/ libasan is reporting some on tests)
• Similar to Thread safety of WriteBatch object not synchronized #6, iterators are also unsafe to call w/o synchronization from the outside: https://github.com/google/leveldb/blob/068d5ee1a3ac40dabd00d211d5013af44be55bea/include/leveldb/iterator.h#L10-L13, add thread safety on those methods.

Note:

• After MAJOR: Update to leveldown v6.1.1 #17, it is unlikely we will get straightforward upstream patches to the bdb from https://github.com/Level/classic-level, so with this PR we fully ditch patches and integrate everything w/o them. Instead we will have to port necessary patches ourselves. (Still quite compatible, so should be easier)

[nodech]

Two memleaks were introduced by this PR and one was old (NewBloomFilterPolicy was not freed). Fixed both

[nodech]

Iterators only method that is actively being used (Next) is behind the nexting_ guard. So it should be safe to use. If we want to make Seek async and add Prev then we will need to generalize this little bit more.