CompactedDBImpl::MultiGet() for better CuckooTable performance

Summary:
Add the MultiGet API to allow prefetching.
With file size of 1.5G, I configured it to have 0.9 hash ratio that can
fill With 115M keys and result in 2 hash functions, the lookup QPS is
~4.9M/s  vs. 3M/s for Get().
It is tricky to set the parameters right. Since files size is determined
by power-of-two factor, that means # of keys is fixed in each file. With
big file size (thus smaller # of files), we will have more chance to
waste lot of space in the last file - lower space utilization as a
result. Using smaller file size can improve the situation, but that
harms lookup speed.

Test Plan: db_bench

Reviewers: yhchiang, sdong, igor

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23673
option to choose module when calculating CuckooTable hash

Summary:
Using module to calculate hash makes lookup ~8% slower. But it has its
benefit: file size is more predictable, more space enffient

Test Plan: db_bench

Reviewers: igor, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23691
improve memory efficiency of cuckoo reader

Summary:
When creating a new iterator, instead of storing mapping from key to
bucket id for sorting, store only bucket id and read key from mmap file
based on the id. This reduces from 20 bytes per entry to only 4 bytes.

Test Plan: db_bench

Reviewers: igor, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23757
reduce memory usage of cuckoo table builder

Summary:
builder currently buffers all key value pairs as a vector of
pair<string, string>. That is too much due to std::string
overhead. It wasn't able to fit 1B key/values (12bytes total) in 100GB
of ram. Switch to use a plain string to store the key/value sequence and
use only 12GB of ram as a result.

Test Plan: db_bench

Reviewers: igor, sdong, yhchiang

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23763
delay initialization of cuckoo table iterator

Summary:
cuckoo table iterator creation is quite expensive since it needs to load
all data and sort them. After compaction, RocksDB creates a new iterator
of the new file to make sure it is in good state. That makes the DB
creation quite slow. Delay the iterator db sort to the seek time to
speed it up.

Test Plan: db_bench

Reviewers: igor, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23775