pkg: update bdb to 1.5.1 and use @handshake-org/bfilter.

[nodech]

• Updated bfilter
  • bfilter has been behind and was using old mrmr, new one uses bcrypto mrmr.
  • Forked https://github.com/bcoin-org/bfilter/ - original is using git dependencies. Until we decide to move everything away from npm, it's better to use npm than mix.
• updated bdb
  • Updated bdb which fixes several segfaults caused by incorrect guards inside it.
  • bdb now has async iterators.
• Fix the test case, which suffered from the previous segfaults by of wdb.close/sync race condition. After bdb update, it manifested as iterators has already ended error.

[coveralls]

coverage: 68.646% (+0.02%) from 68.623% when pulling 1cc63b5 on nodech:update-bdb into 4e87fb1 on handshake-org:master.