Summary:
Currently the point lookup values are copied to a string provided by the user.
This incures an extra memcpy cost. This patch allows doing point lookup
via a PinnableSlice which pins the source memory location (instead of
copying their content) and releases them after the content is consumed
by the user. The old API of Get(string) is translated to the new API
underneath.

 Here is the summary for improvements:
 1. value 100 byte: 1.8%  regular, 1.2% merge values
 2. value 1k   byte: 11.5% regular, 7.5% merge values
 3. value 10k byte: 26% regular,    29.9% merge values

 The improvement for merge could be more if we extend this approach to
 pin the merge output and delay the full merge operation until the user
 actually needs it. We have put that for future work.

PS:
Sometimes we observe a small decrease in performance when switching from
t5452014 to this patch but with the old Get(string) API. The difference
is a little and could be noise. More importantly it is safely
cancelled
Closes https://github.com/facebook/rocksdb/pull/1732

Differential Revision: D4374613

Pulled By: maysamyabandeh

fbshipit-source-id: a077f1a