namedb: support more flags / operations
namedb: interactive iterator (lmdb) + updated tests

Not supported in trie because of the API incompatibility.
namedb: cleanup, comments
namedb: implemented GEQ/LEQ for lmdb backend + tests
Merge branch 'master' into namedb_iter_seek
namedb: implemented clear() operation
Merge branch 'namedb_iter_seek' into 'master'

Namedb iter seek

This merge request extends the namedb api for following operations:
- `clear()` which equals to deletion of all entries in the database
- `iter_seek()` which returns an iterator to given key (or first, last, leq, geq)
  - this is not supported in the trie yet, but it works for lmdb backend
The unittests are extended to cover these operations.

See merge request !302