Minor lmdb related improvements (#667)

* don't call initCursor on happy path

* don't call initCursor on happy path

* don't run stale reads goroutine for inMem mode

* don't call initCursor on happy path

* remove buffers from cursor object - they are useful only in Badger implementation

* commit kv benchmark

* remove buffers from cursor object - they are useful only in Badger implementation

* remove buffers from cursor object - they are useful only in Badger implementation

* cancel server before return pipe to pool

* try  to fix test

* set field db in managed tx