Merge pull request #7052 from branch-predictor/bp-faster-kv-get

kv: implement value_as_ptr() and use it in .get()

Reviewed-by: Sage Weil <sage@redhat.com>

diff --cc src/kv/KeyValueDB.h
Simple merge

diff --cc src/kv/RocksDBStore.cc
Simple merge

diff --cc src/kv/RocksDBStore.h
Simple merge