os/KeyValueDB: don't call self.valid() from next() and prev()

os/KeyValueDB: don't call self.valid() from next() and prev()

In most loops using ObjectMapIteratorImpl, there is a call to valid()
followed by a call to next(), which calls valid() again. Calling
valid() alone is particularly expensive, so don't do it twice if
possible.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>