Summary:
Based on https://github.com/facebook/rocksdb/issues/843
It looks that when the data is hot we spend significant amount of time moving data out of RocksDB blocks. This patch reduce moving memory when possible
Original performance
```
$ time ./ldb --db=/home/tec/local/ellina_test/testdb scan > /dev/null
real 0m16.736s
user 0m11.993s
sys 0m4.725s
```
Performance after reducing memcpy
```
$ time ./ldb --db=/home/tec/local/ellina_test/testdb scan > /dev/null
real 0m11.590s
user 0m6.983s
sys 0m4.595s
```
Test Plan:
dump the output of the scan into 2 files and verifying the are exactly the same
make check
Reviewers: sdong, yhchiang, anthony, rven, igor
Reviewed By: igor
Subscribers: dhruba
Differential Revision: https://reviews.facebook.net/D51093
88e0527724