Close cursors when commit(writer). Factor out code.

Close cursors when commit(writer). Factor out code.

Close remaining cursors when committing a write txn. The doc says this
happens, and it avoids cursor tracking when updating mainDB + freeDB.

Rename mdb_cursor_merge() -> mdb_cursors_close() for code reuse,
and add a merge option.  Simplify its loop a bit.

Factor out cleanup of DBIs.